Extract mrbtest to binary gem

We have do this because mruby's test files are found using MRUBY_ROOT, like this:
    mrbs = Dir.glob("#{MRUBY_ROOT}/test/t/*.rb")

Typo in mruby-bin-debugger/mrbgem.rake

C++11 Build Error: C++11 requires a space between literal and identifiers

Updated string.h.md

Refactor version macros, and add a global const variable

C string.h API documentation.

run bintests within subfolders of bintest/

Add String#freeze test

Removed old mrbgems_test.rake which was merged into mrbgem spec

Use #nil? instead of == nil.

Refactor mrubygem code (range.rb, numeric.rb, string.rb, array.rb, enum.rb)

numeric.rb: refactor code (Avoid using {...} for multi-line blocks,  Surrounding space missing in default value assignment)

range.rb: refactor code (use ! instead of not, use favor modifier if and unless usage when having a single-line body)

array.rb: refactor some code