Warned by gcc 5.3.1 and clang 3.7.0

Warned by gcc 5.3.1

Add Canonicalization

Bka fix union with group in generic struct

Re-enable MSVC lite-mode

kind<T>() is used in the test.capnp.h generated header. MSVC accepts it,
so it probably makes more sense to expose kind<T>() in lite-mode than to
use CAPNP_KIND(T) in the generated header code.

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.

MSVC cannot compile the overloaded member function SFINAE technique used
in Kind_ and FromAny_. It has an easier time with std::void_t-style
SFINAE.

Code which relies on RPC clients and servers cannot compile in lite-mode.

Note that none of the FromAny static_asserts in common-test.c++ will
compile yet in MSVC. Nevertheless, the first three (FromAny<int>,
FromAny<Reader>, FromAny<Builder>) should reasonably work in lite-mode.

Stringification depends on the dynamic API, which is unavailable in lite
mode.

Thanks to @kentonv for the wording.

Bka cleanups

* improves code consistency
* allows to add new case at the end

Adding union into TestGenerics struct leads to compilation error
in generated header:
````cpp
In file included from external/capnproto/c++/src/capnp/test_capnp/capnp/test.capnp.c++:4:0:
external/capnproto/c++/src/capnp/test_capnp/capnp/test.capnp.h:9565:10: error: need ‘typename’ before ‘capnproto_test::capnp::test::TestGenerics<Foo, Bar>::Ug::Reader’ because ‘capnproto_test::capnp::test::TestGenerics<Foo, Bar>::Ug’ is a dependent scope
   inline Ug::Reader getUg() const;
````
Relavant parts in header file:

````cpp
template <typename Foo = ::capnp::AnyPointer, typename Bar = ::capnp::AnyPointer>
struct TestGenerics {
  ...
  struct Ug;
};

template <typename Foo, typename Bar>
class TestGenerics<Foo, Bar>::Reader {
  ...
  inline Ug::Reader getUg() const;
};
````

Compiler misses `typename` keyword before Ug::Reader.

Fixes a spurrious test failure with out-of-tree builds.

Fixes #235
Closes #325

Was incorrect operator!= for Text::Builder

You can now spawn a canonical message from:
* StructReader
* AnyStruct::Reader
* capnpc-c++ generated Foo::Reader

Add a cmake package config module + further cmake cleanups

It is useful for autoconf based installations.

Fix JSON handler for DynamicStruct. The handler needs to be told the struct's schema to construct an orphan.

Also improve error message when addTypeHandler() is given a handler for a dynamic type but no specific schema is specified. We could support this eventually but we don't presently.

Problem discovered and initial test case contributed by Harris Hancock.

miniposix.h shouldn't be executable

autotools build does not have this option.
Capnp tools are built when CAPNP_LITE == OFF