Don't use chrono in snitch headers

As suggested in code review, don't include chrono in the snitch headers
since this increases compile times. Use a basic integral representation
type and then convert back and forth to chrono types in the source files
when actually reading clocks or calculating times.

include/snitch/snitch_section.hpp

@@ -3,9 +3,8 @@
 
 #include "snitch/snitch_config.hpp"
 #include "snitch/snitch_test_data.hpp"
-#if SNITCH_WITH_TIMINGS
-#    include <chrono>
-#endif
+
+#include <type_traits>
 
 namespace snitch::impl {
 struct section_entry_checker {
@@ -16,7 +15,7 @@ struct section_entry_checker {
     std::size_t failures         = 0;
     std::size_t allowed_failures = 0;
 #if SNITCH_WITH_TIMINGS
-    std::chrono::steady_clock::time_point start_time = std::chrono::steady_clock::now();
+    std::make_signed_t<std::size_t> start_time = 0;
 #endif
 
     SNITCH_EXPORT ~section_entry_checker();

src/snitch_section.cpp

@@ -7,8 +7,16 @@
 #if SNITCH_WITH_EXCEPTIONS
 #    include <exception>
 #endif
+#if SNITCH_WITH_TIMINGS
+#    include <chrono>
+#endif
 
 namespace snitch::impl {
+#if SNITCH_WITH_TIMINGS
+using fsec         = std::chrono::duration<float>;
+using snitch_clock = std::chrono::steady_clock;
+#endif
+
 section_entry_checker::~section_entry_checker() {
     if (entered) {
 #if SNITCH_WITH_EXCEPTIONS
@@ -39,12 +47,13 @@ section_entry_checker::~section_entry_checker() {
             // since then we will know if there is any sibling.
             state.sections.leaf_executed = true;
 #if SNITCH_WITH_TIMINGS
-            const auto  end_time = std::chrono::steady_clock::now();
-            const float duration = std::chrono::duration<float>(end_time - start_time).count();
+            const auto end_time = snitch_clock::now().time_since_epoch();
+            const auto duration =
+                std::chrono::duration_cast<fsec>(end_time - snitch_clock::duration{start_time});
             state.reg.report_callback(
-                state.reg,
-                event::section_ended{
-                    data.id, data.location, false, asserts, failures, allowed_failures, duration});
+                state.reg, event::section_ended{
+                               data.id, data.location, false, asserts, failures, allowed_failures,
+                               duration.count()});
 #else
             state.reg.report_callback(
                 state.reg,
@@ -91,7 +100,9 @@ section_entry_checker::operator bool() {
 
         state.sections.levels.push_back({});
     }
-
+#if SNITCH_WITH_TIMINGS
+    start_time = snitch_clock::now().time_since_epoch().count();
+#endif
     ++state.sections.depth;
     asserts          = state.asserts;
     failures         = state.failures;