Add protransform for numeric, varbit, and temporal types

Building on commit 8f9fe6edce358f7904e0db119416b4d1080a83aa, this adds
protransform functions to the length coercions for numeric, varbit, timestamp,
timestamptz, time, timetz and interval. This mostly serves to make more ALTER
TABLE ALTER TYPE operations avoid a rewrite, including numeric(10,2) ->
numeric(12,2), varbit(4) -> varbit(8) and timestamptz(2) -> timestamptz(4).
The rules for varbit are exactly the same as for varchar. Numeric is slightly
more complex:

* Flatten calls to our length coercion function that solely represent
* increases in allowable precision. Scale changes mutate every datum, so
* they are unoptimizable. Some values, e.g. 1E-1001, can only fit into an
* unconstrained numeric, so a change from an unconstrained numeric to any
* constrained numeric is also unoptimizable.

time{,stamp}{,tz} are similar to varchar for these purposes, except that, for
example, plain "timestamptz" is equivalent to "timestamptz(6)". interval has
a vastly different typmod format, but the principles applicable to length
coercion remain the same.

Under --disable-integer-datetimes, I'm not positive that timestamp_scale() is
always a no-op when one would logically expect as much. Does there exist a
timestamp such that v::timestamp(2) differs from v:timestamp(2)::timestamp(4)
due to floating point rounding? Even if so, I'm fairly comfortable calling it
a feature rather than a bug to avoid perturbing values that way.

After these patches, the only core length coercion casts not having
protransform functions are those for "bpchar" and "bit". For those, we could
only optimize trivial cases of no length change. I'm not planning to do so.

Thanks,
nm