Add well-formedness check for weird standalone statements

[jpolitz]

There are a number of expressions that really ought to not appear on their own line. For example,

e1 == e1

especially in a a check block, is probably a mistake. Similarly, a lone identifier, binops, object lookup with dot, and underscore expressions have little reason to appear on their own before the end of a multi-statement block. (Yes, binops could call side-effecting code and be useful... but that's a programming pattern I think we should be happy to disallow).

We should add a well-formedness check to look for these cases. Especially the equality case above should get special treatment, because the user often will have meant e1 is e2.

[blerner]

Relevant: #363

[sorawee]

See also: #410

[shriram]

I'm pleased to note that the original report was not frivolous — the issue clearly has staying power. (-: