These functions are dangerous, as they check whether conversions are possible without actually performing the conversions. In each case where they were used, explorer would crash in some cases if a user-defined conversion is required.
This change moves us more towards implicit conversions being handled by a regular function call on an interface and away from them being magical builtins. Unfortunately, this exposes a pre-existing bug that a call of the form `x.(ImplicitAs(T).Convert)()` compiles even if `x` only has an explicit conversion to `T`. That's worked around here for now, but will need a proper fix later.
Allow explicit conversion from a tuple of `T1`, `T2`, ... to a tuple of `U1`, `U2`, ... if each element has an explicit conversion.
Also add a comment to existing `ImplicitAs` impl for tuples explaining its purpose.
Richard Smith