Verifier computes the buffersize, useful when streaming multiple flatbuffers

Fixed bug with negative hex constants (2).

Close #3898

Don't crash if str is null

Is useful especially when we want to create a string from another message string that might be null.

Rename CONTRIBUTING to CONTRIBUTING.md

Fixed operator++. Added CreateXXX for vector types.

Added --escape-proto-identifiers command-line option.

Fix for issue #3922

Also, clean up redundant ';' at end of java classes.

same fix for general code generator

Fix for make