Re-apply "Introduce FuzzMutate library"

Same as r311392 with some fixes for library dependencies. Thanks to
Chapuni for helping work those out!

Original commit message:

This introduces the FuzzMutate library, which provides structured
fuzzing for LLVM IR, as described in my EuroLLVM 2017 talk. Most of
the basic mutators to inject and delete IR are provided, with support
for most basic operations.

llvm-svn: 311402