Support constants initialization dependency

[byakuren-hijiri]

Consider the following example:

const _0: Int = 1;
const _1: Int = _0 + 1;

Compilation output:

Tact compilation failed
Error: contract.tact:4:16: Cannot reduce expression to a constant integer
Line 4, col 16:
  3 | const _0: Int = 1;
> 4 | const _1: Int = _0 + 1;

Expected: successful compilation as the value of _0 is known in compile time.

Related to #195.

The fix should support the following case:

const x: Int? = 42;
const f: Bool = x == null; // evaluates to false

[anton-trunov]

Resolved in #664