Add missing include when building on Linux

Discovered trying to build on Android using CMake.  UIO_MAXIOV is only
exported by the linux/uio.h header which was never included.  Not sure
how normal Linux builds work.

Add missing reinterpret cast from const void* to const byte*.

Remove -pthread compile flag when building under Android.  ANDROID cmake
variable exported by android-cmake project.

prevent use-after-free in internal compiler API

capnp-c++: implement asGeneric method for outer template contexts

Since simply fixing the bug would silently break backwards-compatibility for affected schemas, we instead throw an exception to alert developers.

See Issue #344 for more details.

https://github.com/sandstorm-io/capnproto/issues/334

fix spelling of "inundating"

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.