fim : redesign cache as ring buffer per key with multi-completion support

- cache now stores up to n_cmpl individual completions per key (ring buffer)
- simplify fim_try_hint: exact match with cycling, nearby match without
- add n_cmpl config option (default: 1)
- add keymap_fim_next/keymap_fim_prev for cycling (<C-J>/<C-K>)
- add llama#fim_cycle() for cycling through completions
- deduplicate completions on cache insert by content
- update info bar to show total cached entries and completion index

Assisted-by: llama.cpp:local pi

Author: ggerganov