support String#[pos, len]= val

fix block variable in Hash#fetch

Don't crash if NULL is passed to mrb_close

target_class should be singleton class of the receiver

Sometimes it is very useful just return from mrb_close if NULL is
passed as mrb.  This is the same spirit of free(3), which just does
nothing if NULL is passed.

Add mrb_utf8_from_locale, mrb_utf8_free, mrb_locale_from_utf8, mrb_locale_free

Add mrb_utf8_from_locale, mrb_utf8_free, mrb_locale_from_utf8, mrb_locale_free. Just works for windows.

Customize installation directory using INSTALL_DIR environment variable

Previously, minirake installed several commands (e.g., mrbc) in
repository locally under bin directory.  But there was no knob for
users to change this directory.  It effectively made `make distcheck`
fail if mruby was embedded into project managed by autotools.

This change adds a way for the user to change installation directory
by setting INSTALL_DIR environment variable.

freeze the hash keys (fixes #2945)

instead origin is saved in ICLASS with MRB_FLAG_IS_ORIGIN set.

Allow rbfiles in mrblib and test to have subdirs

Revert 7b5f8b07 except removing trailing spaces

Added range.h.md and edited re.h.md and version.h.md

Add build_mrbtest after config block is evaluated

This allows us to add `enable_test` anywhere in a build target,
without having to worry about the order in which they are included.

Previously, there was a bug that occured when adding 'mruby-test' gem
to dependencies before additional gems.

Instead of adding the 'mruby-test' gem dependency manually to a test build,
we now only need to call `enable_test` in the target. This also allows us to
call `test_enabled?` downstream when running mruby tests ourselves.

/cc #2924

Added version.h.md and re.h.md