Add dummy feature

[newpavlov]

Closes#49

Strictly speaking it may be considered a breaking change. We could make dummy feature enabled-by-default, but I am not sure if we should.

[josephlr]

This is great! Mostly just grammar/phrasing nits.

[newpavlov]

@josephlr
Thank you for the nits! Feel free to propose any other similar improvements!

[josephlr]

Nice cleanup! Just a few nits. They're essentially just personal preference, so feel free to ignore.

[josephlr]

Personally I find something like:

...}elseif#[cfg(all(target_arch = "wasm32", feature = "wasm-bindgen"))]{#[path = "wasm32_bindgen.rs"]mod imp;} else if #[cfg(all(target_arch = "wasm32", feature = "stdweb"))]{#[path = "wasm32_stdweb.rs"]mod imp;} else if #[cfg(feature = "dummy")]{#[path = "dummy.rs"]mod imp;} else {
compile_error!("\...

to be a little more readable. It avoids nested cfg_if!s, and we don't need the unknown specifier. If we get here, the other wasm32 cases are taken care of.

In theory, a new wasm32 target could be added that's not addressed in the above, but having the stdweb/wasm-bindgen features work in that case seems fine (even ideal).

[newpavlov]

I am not sure about not needing the unknown specifier. Imagine that we don't have WASI or Emscripten and they will be added in future, with this config even if getrandom will work on them, I wouldn't call it a correct target support. But I guess since we can't use target_vendor here, we are not completely future proof anyway, so it makes sense to simplify the code.

[dhardy]

I agree, this is a good way to solve the issue.