Separate implementation for find(CharT ch, size_type pos) overload
is redundant and may be changed to just calling basic implementation
with specified parameters

Separate implementation for rfind(CharT ch, size_type pos) overload
is redundant and may be changed to just calling basic implementation
with specified parameters

Recursion on a long strings may cause stack overflow.
Previous implementation was tricky, as at first glance
rfind( CharT ch, size_type pos = npos ) overload is called and
everything is ok, but it calls
rfind(const CharT *s, size_type pos, size_type count) under the hood.

For consistency use std::distance for pointer arithmetic in every
overload of find

Skip redundant while loop cycles by using traits_type::find return
value, instead of just checking if it finds anything over and over.

Update readmes

Add Dockerfile.ubuntu-rolling

Add Dockerfile.debian-testing

Add Dockerfile.fedora-rawhide

Add Dockerfile.centos-8

.mailmap

Our machines have names spelt inconsistently or not set at all.

Two files have been committed using private email addresses, thus
check-license did not complain.

segment_vector: fix typo and doxygen comments

test: fix briefs of concurrent_hash_map test files

Merge back changes from stable 1.7

Fix array_modifiers test

Move containers out from experimental