Added support for custom gem clone directories when using git
add read barrier to value.p

API changes:
- value.p must be accessed via mrb_value_p macro
- value.p must be mutated via MRB_SET_VALUE_P macro
implement 64bit nan-boxing
cosmetic changes
revert one of the API changes: MRB_SET_VALUE_P no longer used
bugfix: the bit mask was smaller than the size it should be
add commentary to value representation
fix bugs in comment
Improve test of mruby-enum-ext GEM
Improve test of mruby-hash-ext GEM
Improve test of mruby-numeric-ext GEM
Improve test of mruby-object-ext GEM
Improve test of mruby-proc-ext GEM
Improve test of mruby-range-ext GEM
Improve test of mruby-string-ext GEM
Improve test of mruby-symbol-ext GEM
Improve test of mruby-toplevel-ext GEM
Improve test of mruby-objectspace-ext GEM
add "Yuichi Nishiwaki" to AUTHORS
Merge pull request #1435 from Bovi-Li/test-fix-order

[mrbtest] assert_equal(expect, actual) order fix