& function shorthand with template strings

[unlessgames]

Could be handy to be able to write

[0,1,1,2,3,5].map `some template string ${&}` 

[edemaine]

This seems like a natural extension of #480 ("go up a level from a bare &"). Based on the discussion there, we could plausibly even allow for `template ${&}`.toUpperCase() and other trailing content like `template ${&}` + tail (as it seems unlikely you'd want to do these operations on a function).

But where it starts to get ambiguous is if you wanted to write `template ${&.toUpperCase()}`. In the special case of template strings, it seems unlikely that you'd want to pass a function in... but with tag functions it might actually be quite useful. So perhaps this should continue to be processed as-is. (`template ${($) => $.toUpperCase()}`)