add support for building wasm32 target

[dan-da]

Building for wasm32 target was failing, which is problematic for building a downstream crate with proptests that use arbitrary. Both proptest and getrandom crates support wasm32 but they require some convincing to do so. This PR convinces them.

Before

$ cargo build --target wasm32-unknown-unknown
   Compiling getrandom v0.3.3
   Compiling cfg-if v1.0.1
   Compiling zerocopy v0.8.26
   Compiling autocfg v1.5.0
   Compiling fastrand v2.3.0
   Compiling once_cell v1.21.3
   Compiling tempfile v3.20.0
error: The wasm32-unknown-unknown targets are not supported by default; you may need to enable the "wasm_js" configuration flag. Note that enabling the `wasm_js` feature flag alone is insufficient. For more information see: https://docs.rs/getrandom/0.3.3/#webassembly-support
   --> /home/danda/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/getrandom-0.3.3/src/backends.rs:168:9
    |
168 | /         compile_error!(concat!(
169 | |             "The wasm32-unknown-unknown targets are not supported by default; \
170 | |             you may need to enable the \"wasm_js\" configuration flag. Note \
171 | |             that enabling the `wasm_js` feature flag alone is insufficient. \
172 | |             For more information see: \
173 | |             https://docs.rs/getrandom/", env!("CARGO_PKG_VERSION"), "/#webassembly-support"
174 | |         ));
    | |__________^

error[E0425]: cannot find function `fill_inner` in module `backends`
  --> /home/danda/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/getrandom-0.3.3/src/lib.rs:99:19
   |
99 |         backends::fill_inner(dest)?;
   |                   ^^^^^^^^^^ not found in `backends`

error[E0425]: cannot find function `inner_u32` in module `backends`
   --> /home/danda/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/getrandom-0.3.3/src/lib.rs:128:15
    |
128 |     backends::inner_u32()
    |               ^^^^^^^^^ not found in `backends`
    |
help: consider importing this function
    |
33  + use crate::util::inner_u32;
    |
help: if you import `inner_u32`, refer to it directly
    |
128 -     backends::inner_u32()
128 +     inner_u32()
    |

error[E0425]: cannot find function `inner_u64` in module `backends`
   --> /home/danda/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/getrandom-0.3.3/src/lib.rs:142:15
    |
142 |     backends::inner_u64()
    |               ^^^^^^^^^ not found in `backends`
    |
help: consider importing this function
    |
33  + use crate::util::inner_u64;
    |
help: if you import `inner_u64`, refer to it directly
    |
142 -     backends::inner_u64()
142 +     inner_u64()
    |

For more information about this error, try `rustc --explain E0425`.
error: could not compile `getrandom` (lib) due to 4 previous errors
warning: build failed, waiting for other jobs to finish...

After

$ cargo build --target wasm32-unknown-unknown
   Compiling unicode-ident v1.0.18
   Compiling proc-macro2 v1.0.95
   Compiling wasm-bindgen-shared v0.2.100
   Compiling bumpalo v3.18.1
   Compiling log v0.4.27
   Compiling wasm-bindgen v0.2.100
   Compiling getrandom v0.3.3
   Compiling once_cell v1.21.3
   Compiling cfg-if v1.0.1
   Compiling zerocopy v0.8.26
   Compiling autocfg v1.5.0
   Compiling bitflags v2.9.1
   Compiling regex-syntax v0.8.5
   Compiling quote v1.0.40
   Compiling syn v2.0.103
   Compiling num-traits v0.2.19
   Compiling lazy_static v1.5.0
   Compiling unarray v0.1.4
   Compiling arbitrary v1.4.1
   Compiling ppv-lite86 v0.2.21
   Compiling wasm-bindgen-backend v0.2.100
   Compiling wasm-bindgen-macro-support v0.2.100
   Compiling wasm-bindgen-macro v0.2.100
   Compiling rand_core v0.9.3
   Compiling rand v0.9.1
   Compiling rand_xorshift v0.4.0
   Compiling rand_chacha v0.9.0
   Compiling proptest v1.7.0
   Compiling proptest-arbitrary-interop v0.1.0 (/home/danda/dev/neptune/proptest-arbitrary-interop)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 19.66s

[dan-da]

Any idea if this is likely to be merged soon or not?

I have a PR pending for another published crate that depends on this. If its not going to be merged soon we will either need to fork this crate for crates.io or incorporate the functionality into another crate. We prefer to do neither of course.

[jan-ferdinand]

I have forked this crate and published proptest-arbitrary-adapter, where this PR as well as #1 and #2 (to the best of my understanding) are included.

It would be great if there was only one source of proptest-to-arbitrary-truth, but I want wasm-compatibility more than that.