Typecheck void type expressions in return and assignment

[StrongerXi]

Problem

The following 4 programs compile without error, but they obviously shouldn't.

Assigning expression with void type.

void foo() { }

void bar() {
  int x = foo();
}

Return nothing in a non-void type function.

int bar() {
  return;
}

Return void type expression in a non-void function.

void foo() {}

int bar() {
  return foo();
}

Return non-void type expression in a void type function.

void foo() {
  return 1;
}

Proposal

I added exactly 4 small chunks of code in parse.c to handle the cases above accordingly.

Error reporting is also a critical part of compiler, and since the compiler prevents variable declaration with void type already, we should probably handle these cases too. I would even propose to separate out type-checking into a phase, but this really depends on your focus in this project.