[MLton] possible change to annotations

[Stephen Weeks]

Since we already have "local" as a scoping mechanism in MLBs, I was
wondering if it would be reasonable to eliminate the explicit scoping
syntax of annotations, replacing

 <bdec> ::= ann "..." in <bdec> end

with

 <bdec> ::= ann "..."

where the intended meaning is that an annotation applies in the rest
of its scope, just as, e.g., "val x = ..." declares x to be available
in the rest of its scope.

The current syntax is a special case

  ann "..." in <bdec> end
  ===> local ann "..." in <bdec> end