Wasmtime: Implement the custom-page-sizes proposal

[fitzgen]

This commit adds support for the custom-page-sizes proposal to Wasmtime: https://github.com/WebAssembly/custom-page-sizes

I've migrated, fixed some bugs within, and extended the *.wast tests for this proposal from the wasm-tools repository. I intend to upstream them into the proposal shortly.

There is a new wasmtime::Config::wasm_custom_page_sizes_proposal method to enable or disable the proposal. It is disabled by default.

Our fuzzing config has been updated to turn this feature on/off as dictated by the arbitrary input given to us from the fuzzer.

Additionally, there were getting to be so many constructors for wasmtime::MemoryType that I added a builder rather than add yet another constructor.

In general, we store the log2(page_size) rather than the page size directly. This helps cut down on invalid states and properties we need to assert.

I've also intentionally written this code such that supporting any power of two page size (rather than just the exact values 1 and 65536 that are currently valid) will essentially just involve updating wasmparser's validation and removing some debug asserts in Wasmtime.

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Label Messager: wasmtime:config

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

• [x] 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. ```

• [x] 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.

• [x] 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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[fitzgen]

I believe all review feedback has been addressed at this point.

[fitzgen]

oh my god this CI failure is driving me crazy:

https://github.com/bytecodealliance/wasmtime/actions/runs/9489825551/job/26152011334?pr=8763#step:5:243

 error: unused import: `prelude::*`
  --> crates\wasmtime\src\runtime\vm.rs:14:5
   |
14 |     prelude::*, DefinedFuncIndex, DefinedMemoryIndex, HostPtr, ModuleInternedTypeIndex, VMOffsets,
   |     ^^^^^^^^^^
   |
   = note: `-D unused-imports` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(unused_imports)]`

If I remove that import, then all the err2anyhow() calls break. I've tried with all sorts of different cfgs and I can't reproduce locally, even when I copy the ./ci/run-tests.sh invocation from CI.

I have no idea why it thinks this is unused, AFAICT, it is definitely used.