Only instantiate coders once (#50)

- Avoids creating new coder instances on every `save` and `load` call by memoizing them on the runner
- `FileCache#read` and `FileCache#write` also reuse the same instances via a private `coder_for` helper rather than calling `.new` inline
- Some opportunistic renames:
  - `Cache#data` -> `Cache#content`
  - `Coder#save` -> `Coder#generate`
  - `Coder#load` -> `Coder#mount`