Streamline `rustc_span::HashStableContext`.
Currently this trait has five methods. But it only really needs three.
For example, currently stable hashing of spans is implemented in `rustc_span`, except a couple of sub-operations are delegated to `rustc_query_system`: `def_span` and `span_data_to_lines_and_cols`. These two delegated sub-operations can be reduced to a single delegated operation that does the full hash computation.
Likewise, `assert_default_hashing_controls` depends on two delegated sub-operations, `hashing_controls` and
`unstable_opts_incremental_ignore_spans`, and can be simplified.
I find the resulting code simpler and clearer -- when necessary, we do a whole operation in `rustc_query_system` instead of doing it partly in `rustc_span` and partly in `rustc_query_system`.
r? @cjgillot
39219ceb97