use Bison's api.prefix instead of name mangling in expr

use Bison's api.prefix instead of name mangling in expr

This is the equivalent of cc2cb52756f545d6863d6fd801fa23762a81704c. My original
intent was to split the changes that remove the build-time symbol mangling (the
equivalent of 9240da12ab7857975aa34279520f1f6aad113d48) into a separate commit.
However, the expression parser does a lot of messy tricks to collide with and
suppress Bison-generated macros. It was simpler to undo all of this in one go
then try to thread this convoluted needle. Related to #1806.