Add Hash#invert

Refactoring exception test.

Change to use assert_equal explicitly.

Don't reset ridx,eidx. And fix for #1949

Add document of header searcher of compiler.

Add Hash#flatten

Embedded document

Add Hash comments

Fix typo.

Fix comment of exc_inspect (and adjust spaces)

simply use `isfinite`

Add Hash#delete_if

Fix docs path.

Suppress warning when using readline in mirb.

separate mrb_range_beg_len() into two: the one truncates range into the
sequence size, and the one does not.  #values_at uses the latter.

add namespace prefix to `dump_irep`

mruby-math: define and use `Math::DomainError`

MSVC: treat implicit function decl. as error

Fix indent

Delete unused variable in Array#delete_if