Problem: Execution stack is incomplete and inefficient.
Solution: Introduce a proper execution stack and use it instead of
sourcing_name/sourcing_lnum. Create a string only when used.
Problem: Characters below 256 that are not one byte are not always
recognized as word characters.
Solution: Make vim_iswordc() and vim_iswordp() work the same way. Add a test
for this. (Ozaki Kiichi)
