Rename parameter names for mrb_yield

consider gem version to be 0.0.0 unless specified

remove unused variables

Print mrb_value instead of RClass*

fix regexp warning: character class has duplicated range

[mrbgems] GEM Dependency Check

[doc] Add minimal build configuration

[TestEnv] Fallback print method

In relation to #1225.

Adds a minimal build configuration for libmruby which contains as less as possible.

Cleanup hash

Add type check in Hash#merge!

Adding a few more respond_to tests

fix typos.