Previously, this pass would convert any read-only use of a
runtime.stringToBytes call to use the original string buffer instead.
This is incorrect: if there are any writes to the resulting buffer, none
of the slice buffer pointers can be converted to use the original
read-only string buffer.
This commit fixes that bug and adds a test to prove the new (correct)
behavior.
Ayke van Laethem
a896f7f218