This is necessary to get things working in MSVC. Note I swapped unistd.h
for miniposix.h in the compiler, too, which will be necessary to port the
compiler to MSVC.

This commit also pulls capnp/compiler/capnp.c++'s pipe() implementation
details (i.e., 8k reserved memory, _O_BINARY mode) into kj/miniposix.h.

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...

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

Remove 'has' for primitive fields.  Dynamic version always returns true.  This also means that primitives are always included in text format, even if they are default-valued.

Interface code generation.  Also update method grammar to allow multiple returns, and allow specifying request/response structs instead of parameter lists.

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).

Rename non-group again, now to 'slot', which is much nicer.  Also introduce fixed lists as a third kind of field (but not implemented yet).

New method isFoo() exists for union fields only and returns true if foo is the active union field.

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.

Arrange for compiler to send to the code generator the complete contents of all files listed on the command line, all first-level dependencies of those files, and parent scopes of those dependencies, but nothing more.  This means it's sometimes possible to compile a schema file without parsing all transitive dependencies.  Also, in 'decode' mode, don't compile annotations, so that the files defining those annotations need not be opened at all.

Fix compiler bugs.  Now test.capnp compiles to an identical schema to the old Haskell compiler, minus intentional changes.

Fix capnpc-capnp.c++, which appears to have rotted (issue 15).  Should probably write a test at some point...