Support subexpression operator $(...)

    $(2,4,6)
    $(1 + 2)
    $($i = 10; ++$j)
Fix subexpressions not returning $null for assignments.

A subexpression containing an assignment was returning the value of
the assignment as the result of the subexpression when it should have
been returning $null.

These statements now work the same as PowerShell:

    $($i = 1) -eq $null
    $foo = $($i = 1)        # $foo is $null
Merge pull request #227 from mrward/subexpressions

Implement subexpression operator $(...)