Feature-gate electrsd dependency

[gerau]

Describe the feature

Compiling the project that uses smplx for the WASM target (wasm32-unknown-unknown) would fail with the following error:

error: The target OS is "unknown" or "none", so it's unsupported by the errno crate.

The errno crate in the tree is a dependency of the electrsd crate. I'm not sure if electrsd would be needed in the final product, other than in local tests, so it probably would be better to lock it behind dev dependency or feature.

[Arvolear]

Hey, are you trying to compile the lending contracts crate?

[gerau]

Yeah, basically I want to use the lending crate with the features of smplx and expose that to WASM bindings with wasm-bindgen