This applies both two the whole of bare expressions and to any nested
expressions within parentheses. The latter means that an attribute value
can span over multiple lines if it's wrapped in parens:
foo = (
1 + 2 + 3 + 4 + 5
)
This is the first non-trivial expression Value implementation. Lots of
code here, so hopefully while implementing other expressions some
opportunities emerge to factor out some of these details.
