Provide values for enums/constants

When using WIT to describe some existing interface, it is sometimes required
to have enums with specific values. But there is no way to express this in WIT
currently.

Think of bitflags,for example:

we have something simple like this (C)

#define FOO  0x2
#define BAR 0x4

int  some_func(int arg) {
   if (arg == 1)  return FOO; else return BAR;
}

Or your function returns NEGATIVE constants (ERR_X = -1, ERR_Y =-33, ERR_Z = -22)

Or maybe your interface has some magic constants like MY_BETTER_PI=3.44 that needs to be passed around
or may be returned from functions.

I understand that WIT is about TYPES, but plain types are just not enough to describe an interface.
Because currently WIT definition is the only point where your host application can provide this data
to unknown number of its clients in unknown to the host languages.
Otherwise, we are back to days when we had to parse C includes to be able to use FFI.

Start with the WIT type system and existing enum definitions, then compare the requested bitflags, negative error values, and floating-point constants with current syntax and semantics. Done requires a documented design for expressing explicit values and constants in WIT, including how they are exposed to clients.