bug: Value::Integer allows non-safe integers

Discovered in #607 and at least related to #288, it's possible for a non-safe integer that is in the i56 range to slip in as a SmallInteger into Value::Integer, leading to code being too-accurate with integer numbers. Test262 test built-ins/TypedArray/prototype/fill/fill-values-conversion-operations.js shows this issue.

We should replace direct SmallInteger usage with proper Number, BigInt, and PropertyKey APIs.