Previously, the `TVM_SREF_TO_BLOCK`, `TVM_SREF_TO_FOR`, and
`TVM_TYPE_AS` macros required both the input and output variables.
The input variable name is useful for improving the error message
returned, but the output variable name isn't necessary for this
functionality, and prevents the macro from being used as part of an
expression.

* Generate an immediately-invoked lambda expression to allow for an
  independently-scoped `result` variable.

* Use parentheses around the input argument, in case the sref is
  the result of an expression.

* Update all call sites to remove the macro argument providing the
  first argument.