Use Cargo's "namespace features" feature

[alexcrichton]

Cargo recently added the ability to have an optional dependency without
implicitly introducing a new named feature on a crate. This is triggered
with some new directives in the [features] section, specifically:

• The dep:foo syntax means that foo is activated but no implicit
  feature should be added named foo.

• Additionally foo?/bar means that the bar feature of foo is only
  activated if foo is otherwise activated elsewhere, for example a
  conditional activation.

These two features can help avoid extra feature names showing up that we
don't want (e.g. currently the wasmtime crate has a rayon feature)
and additionally can help avoid runtime dependencies in niche cases for
us (e.g. activating all-arch but disabling cranelift would
previously pull-in cranelift but no longer will).

[github-actions]

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