Planned API breaking changes - umbrella issue

[wprzytula]

Not to overlook any planned breaking API change scheduled for before we release 1.0, I group them here.

Tasks

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1. Request execution API changes - umbrella issue #978 - cleaner and more flexible API, more type-safety, possible efficiency gains; mind Optimize CachinSession #477 and Avoid allocations in CachingSession::execute #475;
2. SerializeCql is a confusing name for a macro #905 - multiple users got confused; mind Is it possible to use the new SerializeRow without using a generic type? #890;
3. Re-structure modules for more reasonable layers' and components' separation #977 - so that the structure makes more sense; mind Datacenter struct not exported. #618;
4. Create a prelude module #637 - for smoother user experience, especially at first steps;
5. Move non-critical metadata out of Session #595 - solves metadata refresh is really expensive! #786, enables solving Unify ColumnType and CqlType #691;
6. Switch deserialization to an iterator-based interface to avoid allocations #462 - large deserialization refactor providing new features as well: [Feature] Get row by name #271; mind Parsing a UDT with a field not present in a struct results in a BadCqlType error #432;
7. Better names and higher granularity for errors #519 - rethink errors; mind that in Switch deserialization to an iterator-based interface to avoid allocations #462; mind Make it possible to nest FromCqlValError #702;
8. Don't use LocalQuorum as default consistency for SimpleStrategy keyspaces #662 - consider this and decide if we change it before 1.0;
9. Capacity for serialization is suboptimal #579 - consider this and decide if we change it before 1.0;
10. Fix uses of SerializedValues::with_capacity in the driver code #385 - consider this and decide if we change it before 1.0;
11. Add support for custom CQL types #364 - consider this and decide if we change it before 1.0;
12. Remove types from pre-1.0 crates from the public API, or hide them behind features #771 - revisit the whole public API before release 1.0.