mpk: maintain mapping of pkey ID to stripe ID

[abrown]

Previously, we assumed that the Wasmtime engine would be able to
allocate keys 1-15 from the OS, in that order. (Recall that Linux
reserves key 0 for itself). While enabling various tests for MPK,
tests in wasmtime_fuzzing::oracles would fail because Wasmtime could
only start allocating at key 2, e.g.; it turns out that the
diff_v8::smoke test instantiates V8 which happens to allocate a key
for itself.

The reason for the "allocate keys 1-15 in order" assumption was that the
logic for calculating the stripe each key owned was very simple: key - 1. We needed some way to map each key ID to the stripe ID it is
associated with.

With this change, we maintain a little bit more state in order to make
the mapping less brittle. ProtectionKey stores the "key ID to slice
ID" mapping as an additional u32 in the struct. This means that,
regardless of what other code in the process allocates MPK keys,
Wasmtime should be able to work fine with the remaining keys it can
allocate.

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