Delete unused variable in Array#delete_if
Fix indent
MSVC: treat implicit function decl. as error

As equivalent to GCC's `-Werror-implicit-function-declaration`.
add namespace prefix to `dump_irep`
mruby-math: define and use `Math::DomainError`

Domain errors are detected as per ISO C99 and may differ from CRuby.
Merge pull request #2173 from suzukaze/delete-unuse-variable

Delete unused variable in Array#delete_if
Merge pull request #2174 from suzukaze/fix-indent

Fix indent
Merge pull request #2176 from cremno/msvc-treat-implicit-function-declaration-as-error

MSVC: treat implicit function decl. as error
Merge pull request #2177 from cremno/math-domain-error

mruby-math: define and use `Math::DomainError`
Merge pull request #2178 from cremno/rename-dump_irep

add namespace prefix to `dump_irep`