lib: speed up require(), phase 2

lib: speed up require(), phase 2

Replace calls to fs.readFileSync() with an internal variant that does
not create Error objects on failure and is a bit speedier in general.

A secondary benefit is that it improves start-up times in the debugger
because it no longer emits thousands of exception debug events.

On a medium-sized application[0], this commit and its predecessor reduce
start-up times from about 1.5s to 0.5s and reduce the number of start-up
exceptions from ~6100 to 32, half of them internal to the application.

[0] https://github.com/strongloop/loopback-sample-app

PR-URL: https://github.com/nodejs/io.js/pull/1801
Reviewed-By: Trevor Norris <trev.norris@gmail.com>