Simplify implementation of `Namer::namespace`. (#1438)

Eliminate `Namer::namespace_index` and remove namespace indices from
`Namer::unique` keys. Instead, implement `Namer::namespace` by just swapping in
a fresh table for the duration of the call.

Create the fresh hash table with the right initial capacity. The prior
implementation did manage to avoid allocations by sharing a hash table, and we'd
like to not lose that advantage entirely.

This has no effect on generated code.