added a couple of rewriting utility functions
added flags to enable/disable rewrite multiple instances
one more command line option --to-counter=<num>

When this option is given, clang_delta will
rewrite multiple instances [counter,to-counter]
simultaneously. Note that currently only
replace-function-def-with-decl supports this feature.

--counter=<num> specifies the starting instance
--to-counter=<num> specifies the ending instance
a new transformation --replace-function-def-with-decl

Replace a function's def with its corresponding decl.

This transformation can rewrite multiple instances if --to-counter=<num>
is passed
build replace-function-def-with-decl