Add SequenceValue for heterogeneous sequences

[JelleZijlstra]

PEP 646 requires that we support heterogeneous, variable-length
sequences like tuple[int, *tuple[str, ...]]. This requires replacing
the SequenceIncompleteValue class, so we're doing this in steps:

1. Add SequenceValue and accept it in all places that currently
   accept SequenceIncompleteValue, but don't infer it in any
   existing contexts (this PR).
2. Update internal usages of SequenceIncompleteValue to also accept
   SequenceValue.
3. Infer only SequenceValue inside pyanalyze.
4. Remove internal usage of SequenceIncompleteValue.
5. Drop SequenceIncompleteValue.