Replaced constant fixed-length u8 vectors with byte strings (fixes #3257)

Replaced constant fixed-length u8 vectors in CodegenRust.py with byte strings (fixes #3257).

@jdm said that there is also an occurrence of a fixed-length vector in eventtarget.rs. I found this one, but when I tried to replace it, it gave errors. When I replaced it with `b"..."`, it said:

    dom/eventtarget.rs:182:46: 182:56 error: mismatched types: expected `&'static [i8]` but found `&'static [u8]` (expected i8 but found u8)
And when I did the same but with dropping the `b` (so, by changing it to `"..."`), it said:

    dom/eventtarget.rs:182:46: 182:55 error: mismatched types: expected `&'static [i8]` but found `&'static str` (expected vector but found str)

I don't know whether there are other prefixes for the strings to make this work, but if there are, let me know and I'll use it there.