feat: compile to wasm for browsers (#187)

## Description

Depends on n0-computer/irpc#54 (merged but not
released)

This makes iroh-blobs compile on wasm, with memstore only though. I had
started on this a while back but it got much easier now with the
fs-store behind a feature flag.

Example: 
n0-computer/iroh-examples#139
Deployed at 

https://n0-computer.github.io/iroh-examples/pr/139/browser-blobs/index.html
## Breaking Changes

<!-- Optional, if there are any breaking changes document them,
including how to migrate older code. -->

## Notes & open questions

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-->

---------

Co-authored-by: “ramfox” <“[email protected]”>

Author: Frando

.cargo/config.toml

@@ -0,0 +1,3 @@
+[target.wasm32-unknown-unknown]
+runner = "wasm-bindgen-test-runner"
+rustflags = ['--cfg', 'getrandom_backend="wasm_js"']

.github/workflows/ci.yaml

@@ -277,3 +277,40 @@ jobs:
     - uses: actions/checkout@v5
     - run: pip install --user codespell[toml]
     - run: codespell --ignore-words-list=ans,atmost,crate,inout,ratatui,ser,stayin,swarmin,worl --skip=CHANGELOG.md
+
+  wasm_build:
+    name: Build & test wasm32
+    runs-on: ubuntu-latest
+    env:
+      RUSTFLAGS: '--cfg getrandom_backend="wasm_js"'
+    steps:
+      - name: Checkout sources
+        uses: actions/checkout@v4
+
+      - name: Install Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: 20
+
+      - name: Install stable toolchain
+        uses: dtolnay/rust-toolchain@stable
+
+      - name: Add wasm target
+        run: rustup target add wasm32-unknown-unknown
+
+      - name: Install wasm-tools
+        uses: bytecodealliance/actions/wasm-tools/setup@v1
+
+      - name: Install wasm-pack
+        uses: taiki-e/install-action@v2
+        with:
+          tool: wasm-bindgen,wasm-pack
+
+      - name: wasm32 build
+        run: cargo build --target wasm32-unknown-unknown --no-default-features
+
+      # If the Wasm file contains any 'import "env"' declarations, then
+      # some non-Wasm-compatible code made it into the final code.
+      - name: Ensure no 'import "env"' in wasm
+        run: |
+          ! wasm-tools print --skeleton target/wasm32-unknown-unknown/debug/iroh_blobs.wasm | grep 'import "env"'