pw_chrono: Improve SystemClock C API

Changes the SystemClock C Api to:
1) Use a pw_chrono_SystemClock_Duration struct instead of aliasing
   an int64_t under pw_chrono_SystemClock_TickCount which could
   accidentally permit direct tick usage.
2) Add PW_SYSTEM_CLOCK_{MS,S,MIN,H} and
   PW_SYSTEM_CLOCK_{MS,S,MIN,H}_CEIL to permit C API users to
   create durations which round up to the nearest tick for deadlines
   and timeouts, mirroring std::chrono::ceil.
3) Add PW_SYSTEM_CLOCK_{MS,S,MIN,H}_FLOOR to permit C API users to
   create durations which round down to the nearest tick for oddball
   corner cases, mirroring std::chrono::floor.
4) In order to enable said macros, the system_clock_config.h backend
   config was changed to require the clock period as a preprocessor
   defines instead of a std::ratio<>.
5) Renames pw_chrono_SystemClock_TimeDelta to
   pw_chrono_SystemClock_TimeElapsed to make the argument ordering
   make more sense.
6) Changes existing std::chrono::duration_cast usage to
   std::chrono::ceil and std::chrono:floor to set a good example
   to be explicit on rounding.
7) Renames pw_chrono_SystemClock_TickCountsToNsTruncate accordingly to
   pw_chrono_SystemClock_DurationToNsFloor.

Requires: pigweed-internal:9340
Change-Id: Ia628dceac53f964eda7c4aacd3d790b8b0e92207
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/31280
Commit-Queue: Ewout van Bekkum <ewout@google.com>
Reviewed-by: Wyatt Hepler <hepler@google.com>
diff --git a/pw_thread/sleep.cc b/pw_thread/sleep.cc
index 28313fd..403df3a 100644
--- a/pw_thread/sleep.cc
+++ b/pw_thread/sleep.cc
@@ -17,12 +17,12 @@
 using pw::chrono::SystemClock;
 
 extern "C" void pw_this_thread_SleepFor(
-    pw_chrono_SystemClock_TickCount for_at_least) {
-  pw::this_thread::sleep_for(SystemClock::duration(for_at_least));
+    pw_chrono_SystemClock_Duration for_at_least) {
+  pw::this_thread::sleep_for(SystemClock::duration(for_at_least.ticks));
 }
 
 extern "C" void pw_this_thread_SleepUntil(
     pw_chrono_SystemClock_TimePoint until_at_least) {
   pw::this_thread::sleep_until(SystemClock::time_point(
-      SystemClock::duration(until_at_least.ticks_since_epoch)));
+      SystemClock::duration(until_at_least.duration_since_epoch.ticks)));
 }