Function params are leaking into outer scope

[sophiajt]

Example:

fun foo(x: i64) -> i64 {
    return x
}

println(x)

This is happening because the function predecl typecheck isn't creating a new scope when the variables are getting created.

[sophiajt]

Fixed with f460e34617cd4efeea15f74c67827e59c0a0a35b