Fix #194, #193, #192

This adds an AuthorizerBuilder struct that is used to create an Authorizer. All of the mutable behaviour, like adding facts or executing Datalog rules is moved into the builder, while the authorizer is limited to read-only queries (still requiring self mutability to track execution time). The AuthorizerBuilder is compatible with snapshots, to store and reuse checks and policies. It has a build method taking a token as argument, and a build_unauthenticated for authorization without token.

The builder APIs are alo changing. Before, we had the following:

 let mut builder = Biscuit::builder();	
builder.add_fact(r"right("file1", "read")"#)?;
builder.add_fact(r"right("file2", "read")"#)?;
let token = builder.build()?;

Builders are now constructed like this:

let token = Biscuit::builder()
    .fact(r"right("file1", "read")"#)?
    .fact(r"right("file2", "read")"#)?
    .build()?;