Fix and refactor delayed evaluation flag

[mgreter]

Fixes #2042, #2034 and #2057 (Spec tests sass/sass-spec#856).

This is pretty much a complete rewrite of how we handle delayed values:

// Notes about delayed: some ast nodes can have delayed evaluation so
// they can preserve their original semantics if needed. This is most
// prominently exhibited by the division operation, since it is not
// only a valid operation, but also a valid css statement (i.e. for
// fonts, as in `16px/24px`). When parsing lists and expression we
// unwrap single items from lists and other operations. A nested list
// must not be delayed, only the items of the first level sometimes
// are delayed (as with argument lists). To achieve this we need to
// pass status to the list parser, so this can be set correctly.
// Another case with delayed values are colors. In compressed mode
// only processed values get compressed (other are left as written).

Issue #2042 was never fully working, but its behavior was changed with #1986 from undelayed to delayed. From my experience libsass struggled to emulate these behaviors from the beginning. First I figured out why #2042 was never working. It turns out that we have to only set delayed on certain parsed ast nodes. And whoever calls parse_list must tell it if values on the first level must be delayed (//CC @xzyfer). This is the case for declarations and argument lists (and others), but not for nested lists (with exceptions).

Once implemented pretty much all other stuff related to delayed values broke. So I set out to fix them one by one, and most were easy. The trickiest one revealed itself in the following sass:

.foo {
  eval: foobar(true, ((yellow 3) red), 1/2);
}  

This compiles to (in compressed ❗ mode):

.foo{eval:foobar(true, #ff0 3 red, 1/2)}

It shows that undelaying colors vs divisions are really two different things. Because in the sample above the color got undelayed, while the division did not. Another key code in the PR is that binary expression never get delayed when they contain nested binary expressions. With this logic in place I was able to remove most of the complicated code regarding delayed value handling.

[coveralls]

Coverage increased (+0.08%) to 79.078% when pulling 2c75e9b on mgreter:refactor/delayed-values into 2f6e81a on sass:master.

[drewwells]

👍