Introduce IntervalSet

This is a compact, fast storage for variable-sized sets, typically consisting of
larger ranges. It is less efficient than a bitset if ranges are both small and
the domain size is small, but will still perform acceptably. With enormous
domain sizes and large ranges, the interval set performs much better, as it can
be much more densely packed in memory than the uncompressed bit set alternative.

compiler/rustc_index/src/lib.rs

@@ -7,6 +7,7 @@
 #![feature(let_else)]
 
 pub mod bit_set;
+pub mod interval;
 pub mod vec;
 
 // FIXME(#56935): Work around ICEs during cross-compilation.