don't use str{cpy,cat} in mruby and mrbc

MSVC: fix warnings

The length of each string is known. It should be used.

fix #1442

add "Yuichi Nishiwaki" to AUTHORS

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.

Add travisci badge

muby-time: use mrb_free directly

Custom build path pulls git gems for every build target

[mrbtest] assert_equal(expect, actual) order fix

Set filename like irb

API changes:
- value.p must be accessed via mrb_value_p macro
- value.p must be mutated via MRB_SET_VALUE_P macro