Don't wrap StatementExpressions in IIFE in declaration (#202)

[STRd6]

This is about 50% of #202

Most of the work was refactoring pushRef to be more general assignResults to handle pushing to a collection or assigning to a ref.

Things remaining:

• iteration statement expressions
• if statement expression
• single assignment expression rhs

Much later:

• arbitrarily nested expressions (need to unwrap and maintain refs to keep execution order the same)
• arbitrarily chained assignments (ditto)

[edemaine]

I was wondering how you were going to deal with the precedence of something like x := y++ + switch y ... but now I see those aren't unwrapped, and compiled as before.

Overall, this definitely seems like a nice first step in this direction! The whole process is a little daunting, but maybe we can chip away at most important cases like this until it's all done.

Maybe it's worth adding a test that demos the #202 feature, like:

x := switch y
  <? 'number'
    y+1
  else
    return 'bad'

Note that if ... return of #202 still isn't supported, because we don't seem to allow expressionizing if statements... Personally I'd find that more useful than switch, but hopefully it's not too much harder than what's in this PR. (I guess we should try to parse an IfExpression first, and on failure, parse an IfStatement that worst-case gets IIFE wrapped?)

[edemaine]

This and others could simplify to:

Suggested change

	statement: statement,
	statement,

[edemaine]

parent is set by the makeNode call below.

[edemaine]

If TryExpression etc. no longer exist (I think they got replaced by StatementExpression, which seems like a good unification), should remove them here.

[edemaine]

I was going to suggest the example from #202 be added as a test, but it turns out it doesn't work (still gets wrapped in an IIFE):

compiled :=
  try
    require('@danielx/civet').compile str, options
  catch err
    return "error"

On the other hand, this spelling does work. Maybe you need to skip over whitespace?

compiled := try
  require('@danielx/civet').compile str, options
catch err
  return "error"