Semantic checker should catch use of this in a function call

[mattnibs]

The use of this in a function body does not currently get caught by semantic checker but it ideally should:

$ super -c 'fn foo(s): ( upper(this) ) values foo("hi")'
error({message:"upper: string arg required",on:{s:"hi"}})

[mattnibs]

On second thought we like the way this is but would be nice to have static checking on builtin functions.