[Bug] Dependency of const vars on non-const vars is not rejected

[acoglio]

🐛 Bug Report

It looks like currently const variables may depend on non-const variables.

Steps to Reproduce

The two programs below are accepted by the Leo compiler.

Code snippet to reproduce

// program 1:
function main(x: u8) -> u8 {
    const c = x + 1;
    return c + 1
}

// program 2:
function main() -> u8 {
    let a = 1u8;
    const c = a + 1;
    return c + 2
}```

#### Stack trace & error message

No errors, everything goes through.

## Expected Behavior

In program 1 above, `x` does not have a known value at compile time, so the value of a `const` should not depend on it.

In program 2 above, `a` happens to have a known value at compile time, but it was agreed in a Slack discussion that in general a `const` variable should only depend on other `const` variables and on literals, not on `let` variables.

The type checking/inference code should enforce the constancy of the expressions assigned to `const` variables. This constancy can be checked and calculated analogously to mutability and types -- i.e. every expression has a type, a mutability, and a constancy.

## Your Environment

Leo 1.0.7
Rust 1.48.0
macOS 10.15.7

[acoglio]

Also see proposal #515. If that is accepted and implemented, no action would be required on this bug. (The const variables in programs 1 and 2 above would become let variables.)

[collinc97]

With the implementation of #617 this issue is still relevant.