added replaceAt and replaceAll to MutableStringPiece

added replaceAt and replaceAll to MutableStringPiece

Summary:
To make MutableStringPiece more usable (and make the
implementation of replace* methods easier) I made sure that
all the const methods (like find) work for MutableStringPiece
in a fashion similar to how they work with StringPiece
(work with everything auto convertible to StringPiece). As
a result this change is pretty susbstatial in LOC, even
though now much happens here.

Test Plan: unit tests

Reviewed By: marcelo.juchem@fb.com

FB internal diff: D1420965

Tasks: 4632424