Support floating point hue in hsl() and hsla()

[erxclau]

Browsers support a color like hsl(120.152, 100%, 75%) but this library would incorrectly parse the hue and fallback to black.

Changes parsing the hue as an integer to parsing as a number.

CSS Color Module Level 3 4.2.4 suggests that the hue should be a number (and not strictly an integer):

Hue is represented as an angle of the color circle (i.e. the rainbow represented in a circle). This angle is so typically measured in degrees that the unit is implicit in CSS; syntactically, only a <number> is given.

CSS Color Module Level 4 is more explicit that hue can be a number.

This PR also comments out the parse_integer and parse_list_integer functions (and relevant tests) from src/stream.rs because leaving them in the code produced these warnings during build:

warning: methods `parse_integer` and `parse_list_integer` are never used
   --> src/stream.rs:417:12
    |
109 | impl<'a> Stream<'a> {
    | ------------------- methods in this implementation
...
417 |     pub fn parse_integer(&mut self) -> Result<i32, Error> {
    |            ^^^^^^^^^^^^^
...
447 |     pub fn parse_list_integer(&mut self) -> Result<i32, Error> {
    |            ^^^^^^^^^^^^^^^^^^
    |
    = note: `#[warn(dead_code)]` on by default

Happy to uncomment or remove them completely if that's preferred.

Related to vercel/satori#602

[DJMcNab]

Thanks, this seems reasonable!
I'd strongly suggest that we shouldn't land this with the dead code just commented out. I'd lean towards just removing it entirely.

[erxclau]

@DJMcNab I removed the dead code