lang: funcs: Rename the lookup functions

This will make things more consistent with future use.

test/shell/env0.mcl

@@ -15,7 +15,7 @@ $t = sys.hasenv("TEST")
 $f = sys.hasenv("DOESNOTEXIST")
 
 $env = sys.env()
-$m = maplookup($env, "TEST", "321")
+$m = map_lookup_optional($env, "TEST", "321")
 
 file "${tmpdir}/environ" {
 	state => $const.res.file.state.exists,