Basic handling framework for presentation attributes in Stylo, with handling for font-size and color

This introduces a basic framework for servo's style system to be able
to query the style of presentation attributes which it can then insert
into the cascade. It uses that framework to implement the size and
color attributes on <font>.

There are a number of improvements that can be done on top of this:

 - Implement all other properties
 - Abstractify the ruledata parameter of the mappers using templates or virtual dispatch so that it can be a Servo decl block instead
 - Implement aforementiond abstraction over Servo decl blocks (this obsoletes the code in the first item above, so it might just be better to skip that and directly do this)
 - Replace uses of nsHTMLStyleSheet with an abstract base class containing common elements between Servo and Gecko

I'd prefer for these to be done in separate steps.

components/script/dom/htmlfontelement.rs

@@ -123,7 +123,7 @@ impl HTMLFontElementLayoutHelpers for LayoutJS<HTMLFontElement> {
 }
 
 /// https://html.spec.whatwg.org/multipage/#rules-for-parsing-a-legacy-font-size
-pub fn parse_legacy_font_size(mut input: &str) -> Option<&'static str> {
+fn parse_length(mut input: &str) -> Option<specified::Length> {
     // Steps 1 & 2 are not relevant
 
     // Step 3
@@ -153,8 +153,8 @@ pub fn parse_legacy_font_size(mut input: &str) -> Option<&'static str> {
 
     // Steps 6, 7, 8
     let mut value = match read_numbers(input_chars) {
-        (Some(v), _) => v,
-        (None, _) => return None,
+        (Some(v), _) if v >= 0 => v,
+        _ => return None,
     };
 
     // Step 9
@@ -165,18 +165,5 @@ pub fn parse_legacy_font_size(mut input: &str) -> Option<&'static str> {
     }
 
     // Steps 10, 11, 12
-    Some(match value {
-        n if n >= 7 => "xxx-large",
-        6 => "xx-large",
-        5 => "x-large",
-        4 => "large",
-        3 => "medium",
-        2 => "small",
-        n if n <= 1 => "x-small",
-        _ => unreachable!(),
-    })
-}
-
-fn parse_length(value: &str) -> Option<specified::Length> {
-    parse_legacy_font_size(&value).and_then(|parsed| specified::Length::from_str(&parsed))
+    Some(specified::Length::from_font_size_int(value as u8))
 }