Refactor how inlining is configured in Wasmtime

[alexcrichton]

Fold the inlining boolean and the inlining_intra_module options into a single Inlining enum option. This option encompasses both and has an additional mode which is "only intrinsics" where inter-module and intra-module calls are never inlined, but intrinsics for Wasmtime are allowed to be inlined. This is inspired from discussion on #13214 and after some performance work and/or confirmations the expectation is to turn the default inlining mode to Inlining::Intrinsics.

[alexcrichton]

A few points of note here:

• This PR is not seeking to change the default of "inlining is off"
• This PR changes the meaning of -Cinlining to "inline everything you can" as opposed to the previous WhenUsingGc option.

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