Returns the element type of
the most specialized array representation capable of
holding items of the type denoted by typespec.
The typespec is a subtype of
(and possibly type equivalent to)
If typespec is bit,
the result is type equivalent to bit.
If typespec is base-char,
the result is type equivalent to base-char.
If typespec is character,
the result is type equivalent to character.
The purpose of upgraded-array-element-type is to reveal how
an implementation does its upgrading.
The environment is used to expand any derived type specifiers
that are mentioned in the typespec.