feat(wasm-support): add wasm support (#351)

Hi @Eugeny,

As mentioned in the previous pull request, it seems like you would
prefer a complete PR for the entire integration. Hence, I have merged
the previous two pull requests into one, containing the full
integration. Please take a look!

### Key Changes:
1. Introduced the `russh-util` crate, and sealed the runtime with
conditional compilation: **Tokio** for non-WASM, and **wasm-bindgen**
for WASM.
2. Sealed the time functionality in `russh-util`: using **std::time**
for non-WASM, and **chrono** for WASM.
3. Updated `cryptovec`, encapsulating the differences for `memset`,
`memcpy`, `mlock`, and `munlock` into separate files, gated behind
feature flags.
4. Conditionally excluded the `server` module from compiling for WASM.
If anyone needs it in the future, they can follow a similar approach to
the one I used.
5. Restructured the `known_hosts` module and gated it behind a
compilation flag, as the WASM environment typically lacks a file system.
6. Made various minor changes to the project structure to improve
maintainability.
7. Added `server`-specific `cfg` and `cfg_attr` attributes to handle
platform differences in usage.

Let me know if any further modifications are needed!

---------

Co-authored-by: Eugene <inbox@null.page>

.github/workflows/rust.yml

@@ -27,6 +27,18 @@ jobs:
       with:
         package: russh
 
+  Build-WASM:
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v2
+
+    - name: Install target
+      run: rustup target add wasm32-wasip1
+
+    - name: Build (all features enabled)
+      run: cargo build --verbose --target wasm32-wasip1 -p russh
+
   Formatting:
     runs-on: ubuntu-latest
 

Cargo.toml

@@ -1,5 +1,6 @@
 [workspace]
-members = ["russh-keys", "russh", "russh-config", "cryptovec", "pageant"]
+members = ["russh-keys", "russh", "russh-config", "cryptovec", "pageant", "russh-util"]
+resolver = "2"
 
 [patch.crates-io]
 russh = { path = "russh" }

README.md

@@ -75,6 +75,7 @@ This is a fork of [Thrussh](https://nest.pijul.com/pijul/thrussh) by Pierre-Éti
 ### Panics
 
 * When the Rust allocator fails to allocate memory during a CryptoVec being resized.
+* When `mlock`/`munlock` fails to protect sensitive data in memory.
 
 ### Unsafe code
 

cryptovec/Cargo.toml

@@ -15,3 +15,6 @@ libc = "0.2"
 
 [target.'cfg(target_os = "windows")'.dependencies]
 winapi = {version = "0.3", features = ["basetsd", "minwindef", "memoryapi"]}
+
+[dev-dependencies]
+wasm-bindgen-test = "0.3"