Instead of treating `Core.Int` as the toolchain's builtin `IntType`,
model it as a class that adapts the builtin type. This aligns us better
with the intended language model, gives an associated library for
`impl`s involving `Core.Int` to live within, and opens the door adding
member functions to `Core.Int` if we decide that is desirable.
Remarkably it also seems to make the formatted SemIR a little smaller,
because a call to a generic class generates less IR than a call to a
function.
Such indexing operations are created by array initialization. Since we
switched integer literals to be of type IntLiteral we've been attempting
to index arrays with the (empty) representation of an IntLiteral rather
than with an actual integer value.
Richard Smith