[process-launcher] Add a process launching binary

This binary exposes a FIDL process launching service. Nothing currently
runs the process.

Change-Id: I8b60a5ba40ec3f4f406855c53f59e8c178122047

[fidl] MessageBuffer: Avoid possible overflow and check malloc ret val.

We promote the two 32-bit operands to 64 bits early in order to avoid overflow.
The result was already being promoted to 64 bits.

When malloc returns NULL, failing with a specific error makes debugging easier
(vs seeing a segmentation fault later on when the buffer is used).

Part of the security review of core FIDL 2 code.

SEC-41 # comment: done

Change-Id: Ic9d2c8ebafa111e427ac7f1e6fc923ad4fcd426b

[fidl] Move public fidl headers to match naming convention

The fidl library provides headers intended for general use, so the
include path should be spelled <lib/fidl/foo.h> as per
https://fuchsia.googlesource.com/docs/+/master/development/languages/c-cpp/naming.md.
This moves the headers into the proper location and updates includes to
match.

Change-Id: I9db162c3fd40170d63730bc9b7079c7e7e950982

[fidl] Factor MessageBuffer out of MessageBuilder

Turns out the memory management of MessageBuffer is useful on the
receiving side as well.

Change-Id: I37d12c489c16f872e9e12afdd675043f1513aa63