refactor host.rs

[xlc]

Part of #2948

Break host.rs into multiple smaller files to improve maintainability.
No logic change.

[github-actions]

twiggy diff report

Difference in .wasm size before and after this pull request.

---

 Delta Bytes │ Item
─────────────┼────────────────────────────────────────────────────────────────────────────────────────
      -46751 ┊ smoldot::executor::host::ReadyToRun::run_once::hf7ef020065c87327
      +45257 ┊ smoldot::executor::host::host_vm::ReadyToRun::run_once::h60d91872e8e2d959
       -2793 ┊ smoldot::executor::host::HostVmPrototype::from_module::{{closure}}::h672bf0dc81209cf1
       +2512 ┊ smoldot::executor::host::host_function::HostFunction::by_name::h33f47d0c2db81356
       +2228 ┊ smoldot::executor::host::HostVmPrototype::run_vectored::hbc774fcdc3f12576
       -2228 ┊ smoldot::executor::host::HostVmPrototype::run_vectored::hcd2b38832fd0269a
       -2098 ┊ smoldot::executor::host::HostVmPrototype::from_module::h8b3c657c9096b3ed
       +2098 ┊ smoldot::executor::host::HostVmPrototype::from_module::h9501be0f088301dd
       +1644 ┊ <&T as core::fmt::Display>::fmt::h4813082a923ecb43
       -1644 ┊ <&T as core::fmt::Display>::fmt::hd733813346b894c5
       +1516 ┊ smoldot::executor::host::host_function::HostFunction::num_parameters::hf86a14afc5cc5c07
       -1348 ┊ smoldot::executor::vm::interpreter::InterpreterPrototype::new::h0893ed237994f7d6
       +1348 ┊ smoldot::executor::vm::interpreter::InterpreterPrototype::new::hd280adba8850c2bb
       -1194 ┊ smoldot::executor::host::HostVmPrototype::run_vectored::h52dd9330e658574c
       +1194 ┊ smoldot::executor::host::HostVmPrototype::run_vectored::he9817348dbe1b043
       -1117 ┊ <smoldot::executor::host::Error as core::fmt::Debug>::fmt::h314eed3af6e93936
       +1117 ┊ <smoldot::executor::host::host_vm::Error as core::fmt::Debug>::fmt::h4fd00ba9c1fbdf7f
        +986 ┊ smoldot::executor::host::HostVmPrototype::run_vectored::h07076576617c1541
        -986 ┊ smoldot::executor::host::HostVmPrototype::run_vectored::h69ed214ab215bbed
        -979 ┊ smoldot::executor::host::HostVmPrototype::run_vectored::h5a72de80971a37d6
       +1400 ┊ ... and 206 more.
        +863 ┊ Σ [226 Total Rows]