src: replace assert() with CHECK()

Mechanically replace assert() statements with UNREACHABLE(), CHECK(),
or CHECK_{EQ,NE,LT,GT,LE,GE}() statements.

The exceptions are src/node.h and src/node_object_wrap.h because they
are public headers.

PR-URL: https://github.com/node-forward/node/pull/16


Reviewed-By: Fedor Indutny <fedor@indutny.com>