Minor ergonomic improvements to pooling config

[alexcrichton]

• Change Config::allocation_strategy to take impl Into<...> to avoid importing InstanceAllocationStrategy.
• Add PoolingAllocationConfig::new as an alias for ::default

[github-actions[bot]]

Label Messager: wasmtime:config

It looks like you are changing Wasmtime's configuration options. Make sure to complete this check list:

• [ ] If you added a new Config method, you wrote extensive documentation for it.

  Our documentation should be of the following form: ```text Short, simple summary sentence. More details. These details can be multiple paragraphs. There should be information about not just the method, but its parameters and results as well. Is this method fallible? If so, when can it return an error? Can this method panic? If so, when does it panic? # Example Optional example here. ```

• [ ] If you added a new Config method, or modified an existing one, you ensured that this configuration is exercised by the fuzz targets.

  For example, if you expose a new strategy for allocating the next instance slot inside the pooling allocator, you should ensure that at least one of our fuzz targets exercises that new strategy. Often, all that is required of you is to ensure that there is a knob for this configuration option in [`wasmtime_fuzzing::Config`][fuzzing-config] (or one of its nested `struct`s). Rarely, this may require authoring a new fuzz target to specifically test this configuration. See [our docs on fuzzing][fuzzing-docs] for more details.

• [ ] If you are enabling a configuration option by default, make sure that it has been fuzzed for at least two weeks before turning it on by default.

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