Support WebAssembly

[dullbananas]

Error when compiling to wasm32-unknown-unknown:

   Compiling rc-zip v0.0.1
error[E0277]: the trait bound `std::fs::File: ReadAt` is not satisfied
  --> /usr/local/cargo/registry/src/github.com-1ecc6299db9ec823/rc-zip-0.0.1/src/reader/read_zip.rs:64:36
   |
64 |         ReadAt::read_zip_with_size(self, size)
   |                                    ^^^^ the trait `ReadAt` is not implemented for `std::fs::File`
   |
   = note: required for the cast to the object type `dyn ReadAt`