(PUP-4483) Enable Struct keys to be NotUndef or Optional

Prior to this commit, a Struct key was a non empty string literal
and it's type was assumed to always be optional. A consequence of
this was that there was no way to describe a type that required
an entry where the key was required but the value was optional. In
short, there was no way to differntiate a "missing entry" from
"entry with undef value".

This commit changes this so that the key of a struct member is a
proper type. The type is restricted in that it must be possible to
infer a non-empty string from it (this becomes the former name of the
key).

The old behavior is preserved so that if a string literal is used as
a key, it will be converted to its corresponding String type. The
String will then be wrapped in an Optional type if the type of the
value is assignable from Undef.