Add child tries support in host.rs

[tomaka]

cc #166

This PR is the first step towards support for child tries in the VM.

While in Substrate child tries support is I believe handled by the "storage layer", in smoldot I prefer to make things clear and explicit.
For example, if the Wasm code reads the storage key :child_storage:default:foo, I don't emit a ExternalStorageGet variant but instead a ExternalStorageRoot variant with "foo" as the trie. Similarly, if the Wasm code calls ext_storage_next_key_version_1 with :child_storage:default:foo, I emit a ExternalStorageNextChildTrie variant.

Only default child storages are supported, as this is the only kind of child tries that exist at the moment (and in the foreseeable future).

After this PR, child tries don't work yet. There is a lot of effort remaining to implement support for them especially in runtime_host.rs (which is the code that handles the storage diff while the call is in progress, including things like storage transactions). However, this PR does a big chunk of the work by designing an API for them.

[github-actions]

twiggy diff report

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

---

 Delta Bytes │ Item
─────────────┼───────────────────────────────────────────────────────────────────────────────────────
      +52267 ┊ smoldot::executor::host::ReadyToRun::run_once::h1c16150c21dac4c1
      -40639 ┊ smoldot::executor::host::ReadyToRun::run_once::h043d454b780bb679
       +2149 ┊ smoldot::executor::host::SignatureVerification::is_valid::h60e8da940c010813
       -2149 ┊ smoldot::executor::host::SignatureVerification::is_valid::hd4550138c0d2cc73
       -1117 ┊ <smoldot::executor::host::Error as core::fmt::Debug>::fmt::h6802949990fa4809
       +1117 ┊ <smoldot::executor::host::Error as core::fmt::Debug>::fmt::h851758aab05a14a6
       -1031 ┊ smoldot::executor::host::Inner::alloc_write_and_return_pointer_size::h02e51662736730bb
       +1031 ┊ smoldot::executor::host::Inner::alloc_write_and_return_pointer_size::h20f5aa4067f4cab1
        -819 ┊ smoldot::executor::host::ExternalStorageGet::resume_vectored::h6100153d7314490a
        +814 ┊ smoldot::executor::host::ExternalStorageGet::resume_vectored::h5b19ed326ba45e6b
        +797 ┊ smoldot::executor::host::ExternalStorageNextKey::resume::h9a3467cc3b307af3
        +792 ┊ smoldot::executor::host::HostVm::into_prototype::h617f2a5958a059b9
        +772 ┊ <smoldot::executor::host::LogEmit as core::fmt::Display>::fmt::h7326e766e1487a05
        -772 ┊ <smoldot::executor::host::LogEmit as core::fmt::Display>::fmt::hbf83331a1b1b4795
        +733 ┊ smoldot::executor::host::Inner::alloc_write_and_return_pointer_size::h7ff358985579e8b6
        +723 ┊ smoldot::executor::host::Inner::alloc_write_and_return_pointer_size::h65dd9c2780141c13
        -723 ┊ smoldot::executor::host::Inner::alloc_write_and_return_pointer_size::hd09c6130705981ba
        -720 ┊ smoldot::executor::host::HostVm::into_prototype::ha513330f6bb8ca1f
        +704 ┊ data[0]
        +654 ┊ smoldot::executor::read_only_runtime_host::Inner::run::h09faf6768c99a783
       +4737 ┊ ... and 119 more.
      +21760 ┊ Σ [139 Total Rows]

[tomaka]

Note that the design of child tries is a complete clusterfuck full of unspecified corner cases.
It is likely that this PR has some mistakes.
Given the insane amount of time that it would take me to figure out what happens in these corner cases, I'll open an issue afterwards instead.