Fix some const asserts not throwing errors

[cmlsharp]

Currently some assertions that can be statically determined to be false, do not cause a compile time error to be thrown, while some do.

For example,

def main() -> bool:
    assert(2u32 == 3u32)
    return true

causes this error to be thrown

Error: Const assert failed: (no error message given) at
    assert(2u32 == 3u32)
; context:
    assert(2u32 == 3u32)

whereas this essentially identical program does not

def main() -> bool: 
    u32 x = 2
    assert(x == 3)
    return true

(instead circ happily generates a circuit that always returns false)

Needless to say this can make debugging difficult. Luckily, I believe fixing this is a one line change. The culprit seems to be here:

circ/src/front/zsharp/mod.rs

Line 1254 in 2b54efa

match self.expr_impl_::<true>(&e.expression).and_then(|v| {

Since expr_impl is called with IS_CNST = true, it returns Err in the latter program since x is not a const variable. However, as far as I can tell, this check is unnecessary. The relevant matter is whether (after constant folding), the expression evaluates to false which is what the subsequent call to const_bool is checking anyway. Hence the solution seems to be to call expr_impl with IS_CNST = false instead.

[cmlsharp]

Further testing reveals that this change can cause the frontend to slow down substantially on large programs (presumably due to a lot of redundant constant folding, with each call using a fresh, empty cache)