@field builtin function to do field access with a comptime string as the field name

[Hejsil]

Add following builtin function:

@memberTag(comptime T: type, index: usize) -> (error set or enum value from T)

Example of how it works:

@memberTag(enum { A, B, C }, 1) // returns B
@memberTag(error { D, E, F }, 1) // returns E

Idk what the use case is for having it work for enums, but the use case for error sets was mentioned here.

[andrewrk]

Now that I think of it, I think this is accomplished by:

@field(T, @memberName(T, index))

So I'll implement @field instead.