Add return type information to some ext/standard functions

Add return type information to some ext/standard functions

Just the first few functions in basic_functions.c to get started.

If anyone is interested in continuing this, PRs to add type info
will be accepted against master, with the following caveats:

 * Return types only, we are not ready to add argument types yet.
 * Only for functions. For methods the addition of return types is
   BC breaking and needs to be carefully considered.
 * Only in cases where we can actually express the return type, we
   don't have int|false and similar.