lib: reduce util.is*() usage

Many of the util.is*() methods used to check data types
simply compare against a single value or the result of
typeof. This commit replaces calls to these methods with
equivalent checks. This commit does not touch calls to the
more complex methods (isRegExp(), isDate(), etc.).

Fixes: https://github.com/iojs/io.js/issues/607
PR-URL: https://github.com/iojs/io.js/pull/647


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>