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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

skips() advance after the string skipped.

tabs are allowed after `=begin` and `=end`.
raise `SyntaxError` if no =end is found.

the input must rest the length of a string to be peeked at least.
fixes parse error at embedded documents by string eval, and `-e`
command line options.

Domain errors are detected as per ISO C99 and may differ from CRuby.

As equivalent to GCC's `-Werror-implicit-function-declaration`.