Replacement for elements of #96 more specific to Rust implementation style and architecture.

We should update the ADR with the relevant changes from here as well.

Types:

• Make validate_vals_and_commit a method on Commit (Light client: further trait impls, improvements, and tests #84 (comment))
• Use validate_next_vals !
• Introduce a Client type to hold Store and Requester (?) (Light client: bisection, store, requester #100 (comment))
• Put trust_threshold and trusting_period in the Store (?) (Light client: bisection, store, requester #100 (comment))
• Enforce off-by-one issues in the type system, eg.:
  • TrustedState::new taking header h-1 and validators for h (Light client: bisection, store, requester #100 (comment))
  • verify taking vals for the right height (Light client: bisection, store, requester #100)
• make trust_level satisfy Copy or Clone and don't pass by reference (Light client: bisection, store, requester #100)
• memoize results of methods that don't take args, eg. .hash() (Light client impl followup  #96 (comment))
• use an enum for height to get the latest state instead of special-casing 0 (rpc: map height=0 to no parameters in the http call #118)
• introduce a simple event type and let bisection function (or a Client structure it's a method on) take a closure that will fire events. this can be used for logging so we don't need a print statement or logger inside the functions

Module Structure:

• Remove dependency on tendermint types, eg. Hash
• Make it a crate ?
• Rename types.rs to interface.rs (Light client: bisection, store, requester #100 (comment))
• Add example to doc
• Simplify the public API and make it harder/impossible to use incorrectly
• Unexpose verify_single once json tests are updated (525c89f#diff-f0c5bdd887eaf26a3fd9ec5bdc93e2d9R106 and 9fbb749#diff-afc724a3f81219fadc771326ccca3a84R8)

Errors:

• Error::RequestFailed should wrap underlying requester errors (Light client: bisection, store, requester #100 (comment))
• Deduplicate the Error::InvalidSignature with ErrorKind::SignatureInvalid
• Move to an error.rs file
• voting_power_in can't detect signatures from validators not in the current validator set (can't detect signatures from validators not in the current validator set)
• need two distinct errors for InsufficientVotingPower - one when it's insufficient to skip (ie. so do bisection) and one when it's insufficient to verify (ie. return error) Differentiate VotingPower errors #119