capnpc-c++: fix generated code for union inside template

This is blunt: A peer can choose to stop reading new messages from a connection whenever the calls in-flight cross a certain threshold. This is needed in Sandstorm to prevent errant (or malicious) apps from consuming excessive RAM in other parts of the system by flooding them with calls.

KJ_DEFER fix doc: deferred code is run at scope exit

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