into sverker/maint/ets-no-mbuf-trapping/OTP-15660

Many heap fragments do no longer make the GC slow.

Even worse, we are not guaranteed that a yield will provoke a GC
removing the fragments, which might lead to a one-yield-per-bucket
scenario if the heap fragment(s) still remains after each yield.

Avoid traversing all of the list/string when only part of it will be
used. An explicit check that the list is flat is needed since
string:slice() accepts deep lists and more.

* hasse/stdlib/optimize_calendar_rfc3339/OTP-15630:
  stdlib: Optimize calendar:system_time_to_rfc3339()

Not using io_lib for formatting makes the conversion several times
faster.

Long atom names in combination with <c><<>></c> could cause a crash.

Fix bug in binary:encode_unsigned causing a read of uninitialized memory

This patch improves the performance of calendar:gregorian_days_to_date/1
by changing the algorithm for finding the year to log-logarithmic.
The old implementation has linear complexity, which makes function
too slow for large values.
For example:
There is an API that allows you to create events for future dates.
There are users of this API who, for some reasons, choose dates
very far in the future. In such conditions, function works very slow.
New implementation based on interpolation search, takes 1 or 2
iterations at most cases and free from such a flaw.
A unit test was also developed to illustrate the speed of a function
at large values.

The bug could be seen by running the test that is added by this commit
in a valgrind enabled emulator.

Co-authored-by: John Högberg <john@erlang.org>

* dgud/stdlib/unicode-binary-bug/ERL-777/OTP-15428:
  unicode_util did not handle binary input data correctly