std::sizeof is not considered constexpr

[meheff]

std::sizeof is not considered a compile time constant even though the value is known at compile time.

Repro:

import std;

fn foo(x: u32, z:u8) -> u8 {
    x[0:std::sizeof(z)]
}

Error:

std_size_not_const.x:4:20-4:23
0002: 
0003: fn foo(x: u32, z:u8) -> u8 {
0004:     x[0:std::sizeof(z)]
~~~~~~~~~~~~~~~~~~~~~~~~~^-^ TypeInferenceError: Unable to resolve slice limit to a compile-time constant.
0005: }