Add base eval step for map keys while parsing

[mgreter]

Keys need to be immutable for the main structure, so we
need to evaluate them while the map is being parsed. We
may defer the map initialization into the eval step too.

Fixes #1169

[mgreter]

This fixes the issues mentioned in #1169, but there are more cases where we differ from ruby sass. IMO we need to refactor Sass_Value handling internally to streamline all related things. Currenty different parts (binary operations, inspection, equal operator, etc) are scattered around our code base. We should put that all under a streamlined API, but I guess this will not be ready before 3.3.

So this is a start, but as I said we have more issues like:

$colors: (
    "transparent": "string",
    transparent: "color"
);

color { content: map-get($colors, transparent); }
string { content: map-get($colors, "transparent"); }
interpolate { content: map-get($colors, #{trans}#{parent}); }

ruby sass:

color {
  content: "color"; }

string {
  content: "string"; }

interpolate {
  content: "string"; }

libsass:

color {
  content: "color"; }

string {
  content: "string"; }

interpolate {
  content: "color"; }

So we seem to evaluate that string somewhere unnecessarily into a color again ...

[coveralls]

Coverage decreased (-0.13%) to 80.2% when pulling 9c74c7a on mgreter:bugfix/issue_1169 into 66631c0 on sass:master.

[xzyfer]

👍

[AlbertoElias]

That was quick! Thanks :D

[mgreter]

@AlbertoElias Can you confirm that the PR fixes your issues?

[AlbertoElias]

@mgreter I can confirm it fixes it! Cheers!