Mark built-in coercion functions as leakproof where possible.

Making these leakproof seems helpful since (for example) if you have a
function f(int8) that is leakproof, you don't want it to effectively
become non-leakproof when you apply it to an int4 or int2 column.
But that's what happens today, since the implicit up-coercion will
not be leakproof.

Most of the coercion functions that visibly can't throw errors are
functions that convert numeric datatypes to other, wider ones.
Notable is that float4_numeric and float8_numeric can be marked
leakproof; before commit a57d312a7 they could not have been.
I also marked the functions that coerce strings to "name" as leakproof;
that's okay today because they truncate silently, but if we ever
reconsidered that behavior then they could no longer be leakproof.

I desisted from marking rtrim1() as leakproof; it appears so right now,
but the code seems a little too complex and perhaps subject to change,
since it's shared with other SQL functions.

Discussion: https://postgr.es/m/459322.1595607431@sss.pgh.pa.us

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