Put the "rowless" code at the top level, since it's almost as complete as the old code

[elidupree]

What to do with the old code? Delete it? Put it in a subdirectory?

Deleting seems appropriate, considering that we do have the git history. If we want to move it into a subdirectory AND have it continue compiling successfully, it would require rewriting a lot of module paths in use statements. And it certainly seems desirable not to spend time compiling it when we only want to use the new stuff.

To be able to conveniently reference the old code, maybe I should just make a branch at the last commit where the old code still exists in the repo.

This change isn't trivial:

• At the top level, api.rs and api_macros.rs will be deleted, but deterministic_random_id.rs is shared with the new code in its current form, and lib.rs will need edits.
• src/support/collision_detection/ is old, but the other things in src/support/ are compatible.
• src/implementation_support/common.rs has a few reused functions, which should be merged into the current src/rowless/implementation_support/common.rs, but is mostly old. src/implementation_support/insert_only.rs is shared. src/implementation_support/data_structures.rs isn't actually used by the new code at all, but isn't dependent on the old code, so it shouldn't be deleted (but it's not technically proper for it to stay where it is if it doesn't help implement time stewards?) src/implementation_support/list_of_types.rs is old.
• Everything in src/stewards/ is old.
• A bunch of the examples are old.
• The new-API examples will need to be updated to remove rowless:: from all their use statements, but otherwise should work. We also need to update the links in the HTML files after removing "rowless" from some example names.
• Most of the code in src/rowless/ deliberately use relative paths to ease this change.