// Language features.
- Utf8 support.
- Make assert a keyword and disable it on release build.

// To implement.

- implement 'lang.getMaxCallDepth()' (default=1000 like python) and
  setMaxCallDepth(val) to change stack size at runtime. (at: pushCallFrame())

- change or add => to_string() to value.as_string
  and add as_repr, as_bool.

- Implement gdb like debugger (add color print for readability).
- Complete all the TODO; macros.
- implement MAX_ARGC checks (would cause a buffer overflow if not)
  when compiling and calling a function (also in fibers).

// Low priority.
- Ignore line with '\' character.
- Make it possible to override function names.
      - To do so the functions and global variables should be in the same
        buffer as the property of the script.
- Union tagging alter in var.

// Add more.
- Single header for embedding (script in pk, require file IO).
- Complete core methods.
- Complete var methods.
- Complete core functions.
- Complete builtin operators.
- Complete opcodes.
- Complete core libs.
- Complete the docs.
- More Tests.

// Enhancements

- Write a custom hex() function to cast uint64_t to hex
  string (currently our max is uint32_t) and for binary
  (C doesn't have a sprintf("%b", num) for us.

- Ensure that the bitwise operation result `a << b` can be fit
  into a double value before casting it to one. and we're allowing
  the integer overflow when shifting.

// Bugs.
It's at pre-alpha and every thing is left to
implement, and nothing would be work as expected.

- f = func print() end; print(f) # semicolon should be there.
  def f() print() end print(f)   # semicolon should be there.
  make it okay to use semicolon freely like new lines.

- REPL error message should be the first one (not the last).
  >>> def f()
  ...   _f = func()
  ...     _f() // _f not defined (no closure).
  ...   end
  Error: Expected an expression // expected Name '_f' not defined.

  Update: The above code just terminates in repl without printing any
          error messages.