Optimize eval functions with fixed result

[plepe]

When eval functions have a fixed result (e.g. 2+2), pgmapcss uses this fixed result, just like as if the final value would have been given.

E.g.:

node {
  foo: 2+2;
  foo: 4;
}

will both compile into

current['properties']['foo'] = '4'

When using a property where the value is checked/modified before assigning, the check/modification could be done at compile time.

E.g.

way {
  color: red;
  color: concat('r', 'e', 'd');
}

will compile into

current['properties']['color'] = '#ff0000';
current['properties']['color'] = check_color('red');

Therefore, for every map feature which matches this rule the check_color() function will be called, which is not necessary.