CMAKE: remove use of relative paths

When doing out of source builds:

mkdir build
cd build && cmake -G "Unix Makefiles" ../ && make

The build script tripped up on relative paths of /generator/proto and
the files under generator/proto/*.proto.

By using ${PROJECT_SOURCE_DIR}, the paths become absolute and the issue
disappears.

The test case was erroneously comparing whole submsg structures,
which could result in false errors when padding bytes differed.

Thought I was cleaning up the naming by calling the function const_cast(),
because that's what it does. But of course it conflicts with the C++ builtin,
so renaming it to pb_const_cast() now.

Fix infinite loop in pb_check_proto3_default_value

- Occurs with proto3, PB_FIELD_16BIT and submessage > 255 bytes
- Add test case for PB_FIELD_16BIT and proto3

Adds support for proto3 and POINTER field types to have
fixed length bytes arrays. Also changed the .proto option
to a separate fixed_length:true, while also supporting the old FT_INLINE
option.

Restructured the generator and decoder logic to threat the inline
bytes fields more like "just another field type".

Max_size is the allocated size, so users had to add +1 for
the null terminator. Max_length does the +1 automatically
in the generator.

Fix documentation for protoc --plugin argument

WIP: cmake cleanup to support installable host tooling

cmake: Fix library name

Signed-off-by: William A. Kennington III <wak@google.com>