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>

The produced static library should be `libprotobuf-nanopb.a`
instead of the current `liblibprotobuf-nanopb.a`.

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

Fix closing a non-empty substream resulting in an incorrect stream state

Previously the field iterator logic didn't know whether two
oneof fields were part of the same union, or separate. This
caused wrong pointers to be calculated if multiple oneofs were
inside a single message.

This commit fixes this by using dataoffset of PB_SIZE_MAX to
indicate union fields after the first field.

Theoretically PB_SIZE_MAX is also a valid value for data offset,
which could cause errors. Adding a compile-time assert for this
is somewhat difficult. However I consider it extremely unlikely
that there is any platform that could trigger this situation, as
it would require 255 bytes of extra data/padding between two protobuf
oneof fields. On 64-bit architectures the worst case is 16 bytes,
and even esoteric platforms only align to 64 bytes or so. Manual
modification of the generated .pb.h file could trigger this, but
even then it would require pretty bad luck to happen.

Make pb_decode_varint32 public API

Changed to use simple indexing instead of while (count--) in buf_read()/buf_write(),
because the count overflowed from 0 to max on the last iteration. While the unsigned
integer overflow is defined and behaviour was correct, making this simple change
allowed enabling the sanitizer which might catch true errors elsewhere in the code.

Mainly to clean up the code. Also fixed handling of
PB_LTYPE_FIXED_LENGTH_BYTES for proto3 files.

If the type is string, do not try to deference it as int16, int32 or int64.
This may lead to unalign memory access, which may cause trap on some architectures (ARM)

Google-Bug-Id: 28000875
Signed-off-by: William A. Kennington III <wak@google.com>
Change-Id: I1bffd39168abe04593588291b0ebbe5199a00138