Add DELETE statement

No tests yet
Use join-by-space in field-to-sql, join-src-to-sql
Rename select-clauses to query-clauses
Move handling of INDEXED BY to join-src-to-sql
Change representation of DELETE; add test

Replace the :from key with :source, :as, and :indexed-by keys.

This simplifies syntax and lets to-sql be implemented in terms
of join-src-to-sql.

Old syntax:
  (delete (from (src :tbl (as :t) (indexed-by :ix))))

New syntax:
  (delete :tbl (as :t) (indexed-by :ix))
Add syntax tests for DELETE statement
Add INSERT statement and syntax tests

The statement is modeled after sqlite's syntax.
Refactor to-sql for INSERT; add test
Add UPDATE statement and syntax tests
Refactor to-sql for UPDATE; add test
Bump version to v0.2.0

This version includes support for the core data manipulation
statements: select, insert, update, delete.