vim-patch:8.2.0807: cannot easily restore a mapping

Problem:    Cannot easily restore a mapping.
Solution:   Add mapset().
4c9243f9fb

Use MapArgument to reduce number of arguments of map_add().

N/A patches for version.c:

vim-patch:8.2.0809: build failure with small features

Problem:    Build failure with small features. (Tony Mechelynck)
Solution:   Move "expr" inside #ifdef.
5a80f8ad5d

src/nvim/eval.lua

@@ -250,6 +250,7 @@ return {
     map={args=2, base=1},
     maparg={args={1, 4}, base=1},
     mapcheck={args={1, 3}, base=1},
+    mapset={args=3, base=1},
     match={args={2, 4}, base=1},
     matchadd={args={2, 5}, base=1},
     matchaddpos={args={2, 5}, base=1},