Use EMPTY_BYTES in WireFormatNano

Integrated changes from Google opensource branch.

Implement maps for JavaNano

Ruby implementation Map#inspect should be consistent with Hash#inspect

Properly support maps in Ruby protoc and another bugfix.

Previously, we supported map fields in the Ruby DSL. However, we never
connected the final link in the chain and generated `map` DSL commands
for map fields in `.proto` files. My apologies -- I had been testing
with the DSL directly so I missed this.

Also fixed a handlerdata-setup-infinite-loop when a map value field's
type is its containing message.

MIPS: remove immediate constraint within CompareAndSwap.

Add java compiled targets and several tests target into .gitignore.

Improper use of immediate constraint is slightly non-optimal with gcc/gnu-as
but may cause breakage with clang.

Add missing LIBPROTOC_EXPORT to ruby generator

Updated to latest upb and added test for JSON map operation.

override the existing one even for message types.