add a compile error for bitwise operator when it is likely misunderstood to be the power operator

[andrewrk]

Inspired by https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90885

If someone were to type this code:

const two_to_the_power_of_16 = 2^16; // incorrect!

Given that it is unlikely someone would actually want a number literal and then xor and a number literal, maybe this should be an error. This would probably be better expressed as 1 << 16 anyway.

• Should we do this?
• Should it be for only base 2? Base 10? ALL YOUR BASE?

[Tetralux]

I'm not entirely sure that "abitrarily" deciding that you cannot use an operator in specific circumstances, that you otherwise can use, is a particularly good idea.
Though, the idea of preventing this mistake in a way you can catch it immediately is a good one.

I'd suggest that if this becomes an error, that there should still be way to do it and that the error message tells you it.

error: constant top-level variable declarations that use '^' must use `@xor(x, y)` or 
`std.math.pow(ResultType, base, exponent)` instead.

error: '^' in constant top-level variable declarations is not allowed.
note:  it can be easily mistaken for `std.math.pow`.
note:  instead, use a non-top-level declaration where you use this value, or explicitly
use `std.math.pow` or `@xor`.

I'm not sure that having @xor is a good idea either though. Feels rather corner-case-y.