Convert a few datatype input functions to use "soft" error reporting.

Convert a few datatype input functions to use "soft" error reporting.

This patch converts the input functions for bool, int2, int4, int8,
float4, float8, numeric, and contrib/cube to the new soft-error style.
array_in and record_in are also converted.  There's lots more to do,
but this is enough to provide proof-of-concept that the soft-error
API is usable, as well as reference examples for how to convert
input functions.

This patch is mostly by me, but it owes very substantial debt to
earlier work by Nikita Glukhov, Andrew Dunstan, and Amul Sul.
Thanks to Andres Freund for review.

Discussion: https://postgr.es/m/3bbbb0df-7382-bf87-9737-340ba096e034@postgrespro.ru