refactor: simplify string.render and clients.

* lib/std/base.lua (render): Use `cb` suffix for callback
parameter names.
Rename j and w locals to k_ and v_, as they represent the values
of k and v resp. from the previous iteration.
Remove unrelated Haskell comments.
* lib/std/string.lua (prettytostring): Likewise.
* lib/std/container.lua (__tostring): Manually unroll render
invocation with base.tostring functions plugged in.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>