Meta-issue: Changes before next release

[okamsn]

We want these changes before the next release:

• [x] Do more efficient initialization of iteration variables (#142, #144).
• [x] Deprecate :result-type (#154). Use finally-return instead.
• [x] Deprecate :init (#146). Use with instead.
• [x] Unify tests (#128, #144)
• [x] Make always, never, and thereis consistent with other commands (#145).

Things we probably want:

• [ ] Split seq and sequence. seq should use seq.el. sequence should be only for the basic sequence type (lists, arrays, vectors, strings, etc.) (#150)
• [ ] Deprecate beginning as an alternative for start with :at. Simplifies checks and tests.

Things we might want:

~~- [ ] Deprecate reduce and accumulate in favor of set-accum (#153)? We should have some kind of reduce, but I dislike how, to be consistent with other commands, its argument order differs from cl-reduce and seq-reduce.~~