A compatibility layer is provided to ease migration.

This prevents the compiler from reporting warnings in these headers while compiling application code. Hopefully this will stem the never-ending stream of complaints from people who enable pedantic warnings.

This removes all instances of the zero-length brand dependencies arrays (04a9c84d).

The changes to 'capnp/generated-header-support.h' in [b469079d] need to
be applied to both sides of the `#if CAPNP_LITE` conditional.

Extended discussion:
https://groups.google.com/d/msg/capnproto/lRlWBOglQv4/8-Qo96AcZQIJ

To use, pass --disable-reflection to the configure script.

This produces a smaller runtime library. However, using it for this purpose is not recommended. The main purpose of lite mode is to define a subset of Cap'n Proto which might plausibly compile under MSVC. MSVC still lacks full support for constexpr and expression SFINAE; luckily, most of our use of these things relates to reflection, and not all users need reflection.

Cap'n Proto lite mode inherits its name from Protocol Buffers' lite mode. However, there are some key differences:

- Protobuf generated code included global constructors related to registering descriptors and extensions. For many people, this was the main reason to use lite mode: to get rid of these global constructors and achieve faster startup times. Cap'n Proto, on the other hand, never had global constructors in the first place.

- Schemas are actually still available in lite mode, though only in their raw (Cap'n Proto structure) form. Only the schema API (which wraps the raw schemas in a more convenient interface) and reflection API (which offers a convenient way to use the schemas) are unavailable.

- Lite mode is enabled in an application by defining CAPNP_LITE rather than by specifying an annotation in the schema file. This better-reflects real-world usage patterns, where you typically want to enable lite mode application-wide anyway.

- We do not build the lite mode library by default. You must request it by passing --disable-reflection to the configure script. Before you can do that, you must have a prebuilt Cap'n Proto compiler binary available, since the compiler can't be built without reflection.

- Relatedly, the lite mode library is built with the same name as the full library. This library is not intended to be installed. If anything it should be statically linked. But, mostly the option only exists on non-MSVC platform to give us a way to test that we haven't broken lite mode.

See changes to test.capnp for example.

Currently only supported through generated code, not in the dynamic API. Not exactly sure how to expose this in the dynamic API...

Note that embedded schema structures in generated code are still incorrect.

That is to say, whereas previously we would extend capnp::typeId<T>() to a new type by declaring a specialization of it for that type, now we instead have the type contain a nested class called _capnpPrivate which contains a `typeId` constant.

This is necessary because it is impossible to specialize a template for a type which is itself nested inside a template type. E.g. it's impossible to write a specialization `template <typename T> typeId<Foo<T>::Bar>()`; C++ simply doesn't support this. But with generics, Cap'n Proto will allow types to be nested inside templates, so we need this to work.

1) Apparently mixing member defaults and initializer lists is a C++14 thing.
2) My test script compiles with -Werror which errors out on calls to now-deprecated getDependency().

For portions currently copyright by Kenton (most of it), transfer copyright to Sandstorm Development Group, Inc. (Kenton's company).

The license change is practically meaningless, as MIT and BSD 2-clause are legally equivalent. However, the BSD 2-clause license is sometimes confused for its ugly siblings, BSD 3-clause and BSD 4-clause. The MIT license is more immediately recognizeable for what it is.

Rémy Blank and Jason Choy (the two non-trivial contributors) are on record as approving this change:

https://groups.google.com/d/msg/capnproto/xXDd2HUOCcc/gbe_COIuXKYJ

Revamp generated interface to Object fields.  Now there is an ObjectPointer::{Reader,Builder}.  This will simplify the dynamic API (next change) and also makes it easier to delegate decisions about the object type to a function that doesn't know about the context where the object lives (i.e. by passing an ObjectPointer::Builder).

Factor Pointer{Reader,Builder} out from {Struct,List}{Reader,Builder}.  Will make it easier to deal with Object fields.

Make sure generated code compiles cleanly even with pedantic warnings, since some people enable those.

Unnamed unions WIP.  About to change MemberInfo format, which will require re-bootstrapping the code generator.

Pull TextBlob out of capnpc-capnp and into KJ as kj::StringTree.  Use it in the stringification code to make that code cleaner and avoid using iostreams.

Allow SchemaLoader to use a callback to lazily load schema nodes, including dependencies of loaded nodes.

Enable some useful Clang warnings, fix some bugs it detected.  Especially, eliminate all dynamic initializers.