web environment crate and improvements

[Ivshti]

move the environment implementation for the web browser to a common crate which can be either implemented as a feature to stremio-core or as a separate crate altogether

improvements:

• fetch: optimize deserialization; curently we deserialize once in JS, then serialize to a string buffer to deserialize with serde on the WASM side; can be optimized
• fetch: any response code other than 200 should return an error related to the response code
• storage: consider gzipping storage