decode: Fix compiler issue with gcc-5

* gcc 5.0 and 5.1 appear to take issue with this line and generates the
  following error:

    /home/nitro/tmp/nanopb/pb_decode.c: In function ‘pb_decode_noinit’:
    /home/nitro/tmp/nanopb/pb_decode.c:889:60: error: conversion to ‘uint8_t {aka unsigned char}’ from ‘int’ may alter its value [-Werror=conversion]
                 fields_seen[iter.required_field_index >> 3] |= (uint8_t)(1 << (iter.required_field_index & 7));
                                                                ^
* This seems like a compiler bug, but this workaround is harmless.

Eliminates a warning on protoc 3.0.

This will allow message sizes and enum options to be available
across the include files.

Currently searching for .options files for included files may
not work for all path combinations, this is related to issue #116.
Should probably make a pull request to protoc about that.

In preparation for multi-file support in generator.
No functional changes yet.

Previously unsigned enums would throw errors on decoding if the value
went outside the signed range (issue #164).

Currently only helps for enums defined within the same file, but solving
issue #165 will make it work for multiple files also.

This can be generally useful for saving RAM, but also makes
it easier to test for issue #164.

Specify additional fields for @PlatformIO Registry

PlatformIO Library Registry manifest file

Update issue 155
Status: FixedInGit

Update issue 154
Status: FixedInGit

Update issue 153
Status: FixedInGit

Update issue 148
Status: FixedInGit

Update issue 145
Status: FixedInGit

Update issue 118
Status: FixedInGit

This handles special charaters like in "my-file.proto" properly.
Also use headerbasename instead of full path, so that compiling files
with relative path doesn't create symbols like FOO/BAR/BAZ_MESSAGES.

Update issue 149
Status: FixedInGit

Update issue 147
Status: FixedInGit