lang: funcs: Rename the lookup functions

This will make things more consistent with future use.

examples/lang/env1.mcl

@@ -2,7 +2,7 @@ import "fmt"
 import "sys"
 
 $env = sys.env()
-$m = maplookup($env, "GOPATH", "")
+$m = map_lookup_optional($env, "GOPATH", "")
 
 print "print0" {
 	msg => if sys.hasenv("GOPATH") {

examples/lang/maplookup1.mcl

@@ -2,13 +2,13 @@ import "fmt"
 
 $m = {"k1" => 42, "k2" => 13,}
 
-$found = maplookup($m, "k1", 99)
+$found = map_lookup_optional($m, "k1", 99)
 
 print "print1" {
 	msg => fmt.printf("found value of: %d", $found),
 }
 
-$notfound = maplookup($m, "k3", 99)
+$notfound = map_lookup_optional($m, "k3", 99)
 
 print "print2" {
 	msg => fmt.printf("notfound value of: %d", $notfound),

examples/lang/sendrecv1.mcl

@@ -3,7 +3,7 @@ import "world"
 
 $ns = "estate"
 $exchanged = world.kvlookup($ns)
-$state = maplookup($exchanged, $hostname, "default")
+$state = map_lookup_optional($exchanged, $hostname, "default")
 
 exec "exec0" {
 	cmd => "echo hello world && echo goodbye world 1>&2", # to stdout && stderr

examples/lang/states0.mcl

@@ -2,7 +2,7 @@ import "world"
 
 $ns = "estate"
 $exchanged = world.kvlookup($ns)
-$state = maplookup($exchanged, $hostname, "default")
+$state = map_lookup_optional($exchanged, $hostname, "default")
 
 if $state == "one" or $state == "default" {