Shrink span encoding further
Spans are now stored in a more compact form which cuts down on at least 1 byte per span (indirect/direct encoding) and at most 3 bytes per span (indirect/direct encoding, context byte, length byte). As a result, libcore metadata shrinks by 1.5MB.
I'm not a huge fan of the fairly manual encoding/decoding from bits implemented here. Something like Tokio's pack abstraction (https://github.com/tokio-rs/tokio/blob/master/tokio/src/util/bit.rs) might be desirable to cut down on some of the shifting etc. We might also say that this isn't worth doing :)
I took a look at copying the span encoding we use in memory (described [here](https://github.com/rust-lang/rust/blob/master/compiler/rustc_span/src/span_encoding.rs)). I think the format there makes a lot more sense for in-memory storage where prioritizing a fixed length (i.e., 4 or 8 bytes) is much more important. In metadata, it's much easier for us to have variable-length values, so there's less of a cliff if we don't quite fit. The bit packing scheme there would need changes to fit the varint scheme since it has a lot of all-1s patterns as the "relative offset" form.
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