Show error class name when raise Error
Merge pull request #1982 from ksss/err-msg

Show error class name when raise Error
More tests for String#hex, String#oct as against 9434506035c3fc01de55ac0bc8b75497f8b5df5f
Add API to define cfunc Proc with userdata.
The APIs are defined in mruby-proc-ext so include it before using this API.
See mruby-proc-ext's test code for usage.
This should resolve #1794.
Merge pull request #1984 from mattn/test_hex_oct_underscore

More tests for String#hex, String#oct as against 9434506035c3fc01de55ac0bc8b75497f8b5df5f
Merge pull request #1985 from take-cheeze/cfunc_with_env

Add API to define cfunc Proc with userdata.
Hash function to avoid funcalls if possible
singleton class needs to set __outer__; fixed #1983