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 (because it is not a `u8` vector but a `c_char` vector). 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.

Ah yes, this won't work since we can't call functions in constants. Maybe &%s_name[0] instead?