Simplify the implementations of the to_reg* functions.

Simplify the implementations of the to_reg* functions.

Given the soft-input-error feature, we can reduce these functions
to be just thin wrappers around a soft-error call of the
corresponding datatype input function.  This means less code and
more certainty that the to_reg* functions match the normal input
behavior.

Notably, it also means that they will accept numeric OID input,
which they didn't before.  It's not clear to me if that omission
had more than laziness behind it, but it doesn't seem like
something we need to work hard to preserve.

Discussion: https://postgr.es/m/3910031.1672095600@sss.pgh.pa.us