Serialization of some idents results in redundant spaces

[MatsPalmgren]

There's an open cssom issue about serialization of <custom-ident>. Gecko currently serialize "\\31st" as "\\31 st" which violates the general principle that values should serialize to the shortest form (the space is redundant in this case). It appears it comes from hex_escape which unconditionally adds a space.

CC @emilio