Monotonic rewrite (#874)

* Rework timer_queue and monotonic architecture

Goals:
 * make Monotonic purely internal
 * make Monotonic purely tick passed, no fugit involved
 * create a wrapper struct in the user's code via a macro that then
   converts the "now" from the tick based monotonic to a fugit based
   timestamp

We need to proxy the delay functions of the timer queue anyway,
so we could simply perform the conversion in those proxy functions.

* Update cargo.lock

* Update readme of rtic-time

* CI: ESP32: Redact esp_image: Too volatile

* Fixup: Changelog double entry rebase mistake

---------

Co-authored-by: Henrik Tjäder <henrik@tjaders.com>

rtic-time/CHANGELOG.md

@@ -5,11 +5,18 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 
 For each category, *Added*, *Changed*, *Fixed* add new entries at the top!
 
-## Unreleased
+## Unreleased - v2.0.0
+
 
 ### Added
 
 ### Changed
+- Full rewrite of the `Monotonic` API.
+    - Now split into multiple traits:
+        - `Monotonic` - A user-facing trait that defines what the functionality of a monotonic is.
+        - `TimerQueueBackend` - The set of functionality a backend must provide in order to be used with the `TimerQueue`.
+    - `TimerQueue` is now purely based on ticks and has no concept of real time.
+    - The `TimerQueueBasedMonotonic` trait implements a `Monotonic` based on a `TimerQueueBackend`, translating ticks into `Instant` and `Duration`.
 
 ### Fixed