Use trimmed down deserialization impl for config

This reduces binary size from 10.1MiB (6.2MiB for just rustbuild code)
to 9.7MiB (5.8MiB for just rustbuild code).

This also reduces compile time from ~6.1s for incr recompilation to ~5.6s.

There is still a lot of
unnecessary code due to the toml crate monomorphizing every
deserialization impl 5 times.

Author: bjorn3