Use Self to avoid new elided_named_lifetime warning.

[waywardmonkeys]

Current nightly Rust has a new warning about elided named lifetimes enabled by default:

https://doc.rust-lang.org/nightly/rustc/lints/listing/warn-by-default.html#elided-named-lifetimes

[waywardmonkeys]

Without this:

warning: elided lifetime has a name
   --> read-fonts/src/collections/int_set/bitset.rs:600:49
    |
599 | impl<'a> BitSetBuilder<'a> {
    |      -- lifetime `'a` declared here
600 |     pub(crate) fn start(set: &'a mut BitSet) -> BitSetBuilder {
    |                                                 ^^^^^^^^^^^^^ this elided lifetime gets resolved as `'a`
    |
    = note: `#[warn(elided_named_lifetimes)]` on by default

warning: elided lifetime has a name
  --> read-fonts/src/collections/int_set/input_bit_stream.rs:66:57
   |
44 | impl<'a, const BF: u8> InputBitStream<'a, BF> {
   |      -- lifetime `'a` declared here
...
66 |     pub(crate) fn from(data: &'a [u8]) -> InputBitStream<BF> {
   |                                                         ^ this elided lifetime gets resolved as `'a`

[cmyr]

thanks!