lang: parser, funcs: Take out the built-in history function

It's a normally named function for now, until we think of a common
package to move it into.

Hopefully this makes improving the lexer and parser easier for now. We
can consider bringing it back if needed.

examples/lang/hysteresis1.mcl

@@ -12,8 +12,8 @@ $threshold = 1.5	# change me if you like
 
 # simple hysteresis implementation
 $h1 = $theload > $threshold
-$h2 = $theload{1} > $threshold
-$h3 = $theload{2} > $threshold
+$h2 = history($theload, 1) > $threshold
+$h3 = history($theload, 2) > $threshold
 $unload = $h1 or $h2 or $h3
 
 virt "mgmt1" {