Read the value bounds of a temporal number in its base type

[estebanzimanyi]

The vspan covering holds the value bounds of a temporal number in its
base type, read off the value: tint_min_value/tint_max_value as int,
tbigint_min_value/tbigint_max_value as bigint, and
tfloat_min_value/tfloat_max_value as double. A bigint bound stays
exact above 2^53 and no bound is rounded inward, so the covering stays
conservative and pruning never skips a matching row.

The three number types differ in base type, so a covering gives its
fields per type (byType) where they differ and once for the class
(fields) where they do not. The parser resolves them into the fields of
each type and requires a per-type covering to name exactly the types of its
class. The descriptor schema admits bigint as a field type.