Move strip_implicit_coercions() from optimizer to nodeFuncs.c.

Use of this function has spread into the parser and rewriter, so it seems
like time to pull it out of the optimizer and put it into the more central
nodeFuncs module.  This eliminates the need to #include optimizer/clauses.h
in most of the calling files, demonstrating that this function was indeed a
bit outside the normal code reference patterns.