Rewrite FA1.2.mligo (delete others for now)

This rewrites the FA1.2.mligo example, which did not match the spec.

I think this fixes #185 (closed). It should match the spec now, modulo the annotation questions in my comments on #185 (closed).

I made other various aesthetic changes too. It refuses positive tez and reclaims storage space now when possible.

I did not update the other two yet, so I am removing them for now. We can "transpile" this over by hand later, if we want.