implement Hash.[]
change mrb_sym type from uint16_t to uint32_t
update outdated links to the original program.
Merge pull request #2592 from miura1729/original2

update outdated links to the original program.
refactor Hash generator loop
remove line number from raise message
copy documentation comment from CRuby
Merge pull request #2594 from yasuyuki/hash

Implement Hash[]
to_hash/to_a check in Hash[] should only be done when only one argument is given; ref #2594
const_defined? to check superclasses; ref #2593
change class argument of mrb_const_defined_at from `struct RClass*` to `mrb_value` to make it consistent with mrb_const_defined; ref #2593
`Module#const_defined?` to take second optional argument as CRuby; fix #2593
code reduce by using mrb_get_args("n")
Wrap README.md to 80 columns
Wrap INSTALL to 80 columns
Wrap CONTRIBUTING.md to 80 columns
Merge pull request #2595 from drbrain/80_column_README

Wrap README, etc. to 80 columns
Fix `mrb_get_args` arguments types found by mruby-clang-plugin.
Merge pull request #2596 from take-cheeze/fix_args

Fix `mrb_get_args` arguments type error found by mruby-clang-plugin.
Remove duplicated mrb_print_backtrace() declaration

It is declared in mruby.h.