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.

lang/funcs/history_func.go

@@ -27,9 +27,9 @@ import (
 )
 
 const (
-	// HistoryFuncName is the name this function is registered as. This
-	// starts with an underscore so that it cannot be used from the lexer.
-	HistoryFuncName = "_history"
+	// HistoryFuncName is the name this function is registered as.
+	// TODO: move this into a separate package
+	HistoryFuncName = "history"
 
 	// arg names...
 	historyArgNameValue = "value"