introduce mrb_str_cat_lit() to create strings from C string litrals
use mrb_str_cat_lit() more widely
use mrb_intern_lit instead of mrb_intern_cstr for C string literals
fix bug for get_valid_iv_sym

When sub-string is used in the following way, it does not work well.

o = Object.new
o.instance_variable_set(:@a, 1)
o.instance_variable_defined?("@abc"[0,2]) #=> false (this should be true)
add test for __LINE__ and __FILE__
Merge pull request #1744 from cubicdaiya/feature/introduce_mrb_str_cat_lit

Introduce mrb_str_cat_lit
Merge pull request #1749 from take-cheeze/file_line_test

add test for __LINE__ and __FILE__
Merge pull request #1747 from cubicdaiya/issues/use_mrb_intern_lit

use mrb_intern_lit instead of mrb_intern_cstr for C string literals
Merge pull request #1748 from cubicdaiya/issues/optim_get_valid_iv_sym

fix bug for get_valid_iv_sym
compile assert.rb once
use mrb_str_new_lit instead of mrb_str_new for C string literal
Merge pull request #1752 from cubicdaiya/issues/use_mrb_str_new_lit2

Use mrb_str_new_lit instead of mrb_str_new for C string literal
add test for Kernel#instance_variable_defined?
Merge pull request #1753 from take-cheeze/assert_rb

Compile assert.rb once.
add new function mrb_get_backtrace_at() to get backtrace at ci and pc
fix indent
avoid recursion when method_missing happened in inspect; fix #1746
revert 38e9ce21 and API changed; mrb_get_backtrace() -> renamed to mrb_exc_backtrace since this is a backtrace method implementation of Exception; mrb_get_backtrace_at() -> removed; mrb_get_backtrace() -> added to get backtrace in array (like caller)
Delete `!!Notice!!` intro paragraph from README

It's become misleading since the release of 1.0.0 and the launch of mruby.org.
Merge pull request #1754 from pje/patch-1

Delete `!!Notice!!` intro paragraph from README