Configurable opt-level with RUST_PROTOBUF_OPT env var

Also include -g flag by default
More default values

* implement default values for enums
* fix default values in mut_ operations
mut_ operations are not needed for pritmitive types
Use unwrap_or(...) for primitive types

... instead of unwrap_or_else(|| ...). It makes generated code slightly
easier to read by humand and by compiler.
Add comments about fields to generated file
Only generate for files, listed in file_to_generate

... as required in plugin.proto comments.