Create rpc-types crate that can be compiled for wasm32

[ijerkovic]

The issue here is similar to near crypto for wasm32 issue, the core/primitives depends on rand and therefore can not be included as dependency.

I guess most of the rpc-types are here:
https://github.com/near/nearcore/blob/master/core/primitives/src/views.rs

We copy-pasted the structs for our needs, but in long term an rpc-types crate would be handy

[ijerkovic]

@medicz @chefsale

[chefsale]

cc: @matklad @ilblackdragon @bowenwang1996

[matklad]

cc @miraclx, I think this'll just fall-out naturally from the work we are doing on creating the rpc types/primitives crate.