Evaluator: Fix handling of return statements

[anubh-v]

Previously, return statements were being evaluated but they were not treated correctly during evaluation.

Specifically, there were 2 mistakes:

• When evaluating a sequence of statements, if a return statement is encountered, the subsequent statements should be ignored.

Example:

function foo(x) {
    return 5;
    x + 1;
}

foo(5) should return 5, not 6;

• If the body of a function does not contain a return statement, applying that function should give the undefined value.

Example:

function foo(x) {
    x + 1;
}

foo(5) should return undefined, not 6.