Cleanup in syntax::ext::simpleext

[catamorphism]

Two FIXMEs in simpleext: "/* FIXME: handle embedded types and blocks, at least */" in p_t_s_rec and "FIXME: check duplicates (or just simplify the macro arg situation)" in add_new_extension. I don't pretend to have any understanding of this code, so that's all I'll say.

The first one at least seems to have been added by @marijnh , so assigning to him.

[marijnh]

I never saw this code before either, so I'm going to pass.

[graydon]

dead code, closing