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
add "Yuichi Nishiwaki" to AUTHORS
Merge branch 'full-nan-boxing' of https://github.com/wasabiz/mruby into wasabiz-full-nan-boxing
rename mrb_value_p() to mrb_ptr() since _p means predicate in mruby source
return statement should terminate execution of a method defined by define_method
fix #1442

in the marking root phase, we only marked the root context, but leaving
the current context unmarked. when we execute a fiber, the current
context would be changed and trigger this issue.
Merge pull request #1443 from wasabiz/add-me-to-authors

add "Yuichi Nishiwaki" to AUTHORS
Merge pull request #1444 from Fleurer/issue1442

fix #1442
use intptr_t instead of int64_t for 32bit platforms