* separate panicking `freeze()` and fallible `try_freeze()` so callers can use only what they want (DRY impl)
* domainify `clock` errors
* implement `ConcatStr` (used to concatenate `str`s without `alloc`)
* impl tests against `ConcatStr`

Rename `PLLConfig` -> Rust-idiomatic `PllConfig`

remove 3 magic numbers, fixed compiler warnings (removed HFXCLKOSC code as it was unnecessary)

enable HFXCLK, if necessary; remember previous setting and restore after use; remove 3 magic numbers

Add PLL support for core_clk configuration