On tests listed bellow the reserve method is being called before any
allocation on the object therefore realloc being called without
any previous allocation. Inside realloc _data is being used with the
operator '[]', as it is nullptr at that moment it's an undefined
behaviour.

This patch simply returns a call to alloc if _data is nullptr.

This tests fails on PowerPC with Segmentation Fault because of this
issue:
segment_vector_array_expsize_assign_exceptions_oom_0_none
segment_vector_array_expsize_assign_exceptions_oom_0_memcheck
segment_vector_array_expsize_capacity_exceptions_oom_0_none
segment_vector_array_expsize_capacity_exceptions_oom_0_memcheck
segment_vector_array_expsize_modifiers_exceptions_oom_0_none
segment_vector_array_expsize_modifiers_exceptions_oom_0_memcheck
segment_vector_vector_expsize_assign_exceptions_oom_0_none
segment_vector_vector_expsize_assign_exceptions_oom_0_memcheck
segment_vector_vector_expsize_capacity_exceptions_oom_0_none
segment_vector_vector_expsize_capacity_exceptions_oom_0_memcheck
segment_vector_vector_expsize_modifiers_exceptions_oom_0_none
segment_vector_vector_expsize_modifiers_exceptions_oom_0_memcheck
segment_vector_vector_fixedsize_assign_exceptions_oom_0_none
segment_vector_vector_fixedsize_assign_exceptions_oom_0_memcheck

Signed-off-by: Lucas A. M. Magalhães <lamm@linux.ibm.com>

Fix windows CI build

we want to enable builds, but current queue implementation
is problematic on Windows - it will be fixed along other queue changes.

Radix: concurrent iteration

Iterator operator++/--:
 Removing nodes/leaves which are pointed to be iterators might
 result in operator++/operator-- failures (since nodes/leaves are
 detached from the tree find_leaf/next_leaf functions will not
 find a proper entry).

 Fix this by adding a fallback path which just searches from the top
 (using lower_bound/upper_bound).

internal_bound:
 Do not traverse the tree twice. Instead save the path from first
 traversal to a vector and just iterate over the vector. Before
 returning iterator to the user, we can check if the path changed
 (if any node on the path was modified). If so, the operation needs
 to be retried.

Revert "gha: temporarily disable codecov"

This reverts commit 7a41d10f.

Extend run-build.sh error msg if no build_steps provided

Mpsc queue

* Make implementation header only
* Opaque implementation in namespace
* Use std::atomic instead of custom implementation
* Modify ringbuf test to work with c++ changes

Importi multi-producer single-consumer (MPSC) ring buffer from https://github.com/rmind/ringbuf

Implement defer free for radix_tree

and enable overwrite tests (since runtime_initialize_mt is
available).

which enables single-writer multi-reader concurrency. After calling
runtime_initialize_mt() each assignment (done through assing_val or
insert_or_assign) does not modify the value in-place but rather
allocates new node. Also, erase does not free nodes/leaves immediately
but adds them to a garbage list which can be processed using
garbage_collect.

gha: temporarily disable codecov

gha: temporarily disable windows builds

Update dockers

and clean up images a little.

Various fixes

radix_tree: use std::atomic<tagged_pointer>

add readme to benchmarks and fix dividing by zero in radix bench