kj/io: Make FdOutputStream::write fallback to OutputStream::write.

Bka cleanups - fix compiler warnings in kj tests

Implement method kj::StringPtr:parseAs<NUMBER_TYPE>

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.

This method provides kj-ish interface to parse numbers.

Moves code from capnp json decoder to kj::StringPtr.

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

Fixes #235
Closes #325

Was incorrect operator!= for Text::Builder