Refactor Position

A specified position is now a struct made of two values of different types,
the first one being PositionComponent<X>, and the second one PositionComponent<Y>.

A position component is represented by the new enum PositionComponent<Side>,
with the three values Center, Length(LengthOrPercentage), and
Side(Side, Option<LengthOrPercentage>).

Side keywords are represented by the X and Y enums, which don't include a value
for the center keyword anymore. They are accompanied by the Side trait, which
allows us to determine whether a side keyword is "left" or "top".

This refactor simplified the parsing and serialisation code and exposed bugs in it,
where it would reject valid <position> values followed by arbitrary tokens,
and where it would fail to prefer "left" to "right" when serialising positions
in basic shapes.

tests/unit/style/parsing/mod.rs

@@ -20,6 +20,10 @@ fn parse<T, F: Fn(&ParserContext, &mut Parser) -> Result<T, ()>>(f: F, s: &str)
     f(&context, &mut parser)
 }
 
+fn parse_entirely<T, F: Fn(&ParserContext, &mut Parser) -> Result<T, ()>>(f: F, s: &str) -> Result<T, ()> {
+    parse(|context, parser| parser.parse_entirely(|p| f(context, p)), s)
+}
+
 // This is a macro so that the file/line information
 // is preserved in the panic
 macro_rules! assert_roundtrip_with_context {