Eliminate extfunc/cfgfunc/mlfunc as a concept, and just use 'func' instead.

The entire compiler now looks at structural properties of the function (e.g.
does it have one block, does it contain an if/for stmt, etc) so the only thing
holding up this difference is round tripping through the parser/printer syntax.
Removing this shrinks the compile by ~140LOC.

This is step 31/n towards merging instructions and statements.  The last step
is updating the docs, which I will do as a separate patch in order to split it
from this mostly mechanical patch.

PiperOrigin-RevId: 227540453