Published client To Do

Before publishing client crate on crates.io we need:
1. [x] Ensure that error reporting is adequate and errors can be handled apropriately. Probably get rid of `anyhow::Error`. If even if we leave `anyhow`, we should ensure that it's easy to find out if error is recoverable, if it's network error, etc.
2. [ ] Figure out if it makes sense to move query methods into a trait (async traits are kinda complex, though)
3. [x] Review list of dependencies, currently there are too much of them
1. [x] Also async-std should not require `unstable` feature
2. [x] Some of them, like `whoami` should be moved under a feature gate
4. [ ] Review public exports, hide unneeded things and restructure modules if needed
1. [ ] Move `Sequence` under a feature gate (e.g. `unstable`)
5. [ ] Query arguments refactoring
1. [x] `QueryArgs` trait
2. [ ] `QueryArg` trait
3. [ ] `QueryArg` implementation for all basic types
4. [ ] `QueryArgs` implementation for tuple and container types
5. [ ] `derive(QueryArgs)`
6. [x] Create a Datetime struct to represent edgedb's native datetime type #42
7. [ ] Implement most of the RFC1004
1. [ ] Reconnecting connection
2. [ ] Connection pool
3. [ ] Transactions
4. [ ] Retriable transactions
5. [ ] Transaction configuration/Retry options
6. [ ] Capabilities

These tasks can be postponed:
1. [ ] Look into supporting multiple runtimes #49
2. [ ] Ensure that nothing is re-exported from `edgedb-protocol`, so that we can make breaking changes in latter without disturbing users
1. [ ] Figure out what to do with `Queryable` trait
3. [ ] Take a look if we want to get rid of `snafu` for errors (used only internally)

Previous discussion in edgedb/edgedb-cli#112, #30

In the meantime, anyone can use git version at their own risk (API will probably break every now and then)