Sketch transients for HAMT-based data-structures

[arximboldi]

This PR is the first step towards the full implementation of transients for all HAMT-based data-structures: map and set.

In this PR we do define the interface in two modes:

• Using the special types map_transient and set_transient, which define a mutable interface (similar to the case of vector_transient and alike). This is required to benefit from the optimizations brought about by transients when using a tracing garbage collector.
• Using std::move(x)... when performing an operation on an immutable map. This will incur in the same performance benefit as the former when using reference counting for garbage collection (default) and is the recommended way to benefit from it.

In this PR we define and test the interface. We also include the new interface as part of the existing benchmarks for HAMT insertion. The implementation is however naive, this is, it is based on the purely immutable counterpart, an provides no performance benefit. We will explore different alternatives for optimization in subsequent PRs.

Issue #82

PING @omer-s @harryhk from Meta Inc. who is sponsoring this HAMT-transient implementation project!

[codecov-commenter]

Codecov Report

Merging #208 (2232315) into master (b38c67b) will increase coverage by 0.22%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #208      +/-   ##
==========================================
+ Coverage   91.48%   91.70%   +0.22%     
==========================================
  Files         104      108       +4     
  Lines        9790    10062     +272     
==========================================
+ Hits         8956     9227     +271     
- Misses        834      835       +1     

Impacted Files	Coverage Δ
extra/fuzzer/fuzzer_input.hpp	95.45% <ø> (ø)
test/flex_vector/fuzzed-0.cpp	100.00% <ø> (ø)
test/flex_vector/fuzzed-1.cpp	94.44% <ø> (ø)
test/flex_vector/fuzzed-2.cpp	100.00% <ø> (ø)
test/flex_vector/fuzzed-3.cpp	100.00% <ø> (ø)
test/flex_vector/fuzzed-4.cpp	100.00% <ø> (ø)
immer/map.hpp	97.75% <100.00%> (-0.17%)	⬇️
immer/map_transient.hpp	100.00% <100.00%> (ø)
immer/set.hpp	100.00% <100.00%> (ø)
immer/set_transient.hpp	100.00% <100.00%> (ø)
... and 6 more

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