`@trymatch` to return `Some`

[jariji]

Suppose I am trying to write an ast transformation.

using MacroTools: postwalk

postwalk(expr) do e
  r = @trymatch e begin
    :($f($x)) => Some(x)
    :($g($x)) => Some(x)
  end
  @match r begin
    nothing => e
    Some(x) => x
  end
end

Note Some(x) is necessary because otherwise it is impossible to distinguish between "the value happened to be nothing" and "matching failed". It would be a little nicer if @trymatch did the Some(x) automatically because (1) it is convenient for users who use Some anyway and (2) it will lead users toward correct usage; currently it is too easy to accidentally forget Some which may lose information.