some small tweaks

[symful]

In regards of #185

[symful]

Explanations from top to bottom:

1. Let BindAndCopy overrideable by giving it virtual to let developers make functions perm-binded
2. Set nextContext.outerVars to the function's outerVars in ManuallyPushCall so the function won't lose variables outside of its scope. Useful if the function is from the MiniScript code and the function uses variables outside of its scope.
3. Check temps size before fetch the index to not error'd