Read-only and read-write Database and Transaction

[cmbrandenburg]

This commit unifies the Transaction and ReadonlyTransaction types
into a single Transaction type that is generic with respect to whether
the transaction is read-only or read-write. There are two new type
definitions, ReadOnlyTransaction and ReadWriteTransaction, which
should provide some syntactic sugar for applications.

Similarly, this commit changes the Database type to be generic with
respect to read-only vs read-write access, as well as creating two new
type definitions, ReadOnlyDatabase and ReadWriteDatabase. Read-only
databases only provide non-mutating methods. For example, it will be a
compiler error for an application to call the put method on a
ReadOnlyDatabase.

[cmbrandenburg]

This pull request does not make the Environment type generic with respect to read-only vs read-write access. I think doing so is a good idea because it would close the remaining loophole in the lmdb-rs type system that allows an application to call database-modifying methods on an environment opened as read-only, but it's unclear to me how the EnvBuilder::open method should be adjusted to make this work. (EDIT: We would also need to make the DbHandle type generic.)

Note that this pull request has nothing to do with issue #48. Whereas, this PR deals with read-only vs read-write access, that issue deals with unsafe lifetimes.