Problem: No reduce() function.
Solution: Add a reduce() function. (closesvim/vim#5481)
85629985b7
Needs CHECK_LIST_MATERIALIZE from v8.2.0751 (and range_list_materialize from
8.2.0149).
Move e_reduceempty to funcs.c, as it's only used there. Make it static.
Use tv_blob_len, tv_list_len == 0 for empty checks.
Replace vim_memset(&funcexe, 0, ...) with FUNCEXE_INIT.
Leave li initially undefined (tv_list_first returns NULL if list is NULL).
This patch has a memory leak fixed by v8.2.0882.
d746f5aa41
