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: Expression type is used inconsistently.
Solution: Add "ETYPE_IS" and "ETYPE_ISNOT" as separate enum values. Rename
"TYPE_" to "ETYPE_" to avoid confusion.
Problem: Using "int" for alloc() often results in compiler warnings.
Solution: Use "size_t" and remove type casts. Remove alloc_check(), Vim
only works with 32 bit ints anyway.
Problem: Vim script debugger functionality needs cleanup.
Solution: Move debugger code to a separate file. Add more tests. (Yegappan
Lakshmanan, closes#4285)
