Bugfix Update 2.0.1

[burgholzer]

This PR addresses some of the bugs and obstacles that came up when integrating the new package version into our QFR library.

• ⚡/🍎 all hash functions are refactored so that hashing is platform and standard library independent again (fixing slow performance on macOS)
• ⚡ improve memory handling by reverting from using stack and forward_list to dedicated next pointers that are pre-allocated with the entries
• ⚡ only clear compute tables if anything has been collected by the garbage collection
• ⚡ adjust garbage collection limit depending on the number of remaining entries
• ⚡ reduce initial garbage collection limits
• ⚡ introduce additional 1/sqrt(2) static member in complex table
• ⚡ always check 0, 1/sqrt(2), and 1 first upon accessing the respective buckets in the complex table
• 🐛 add nullptr checks in reference counting methods
• ♻️ moved generation of special DDs (e.g., SWAP) from QFR to DD Package
• 🐛 fix bug in colorFromPhase when a negative phase was encountered
• 🐛 fix misaligned access on ComplexTable::Entry
• 🔥 remove unused GC_INCREMENT
• ✅ allow for some up and downs in code coverage
• ⚡ complex table hash now rounds instead of truncates the calculated value
• 🎨 number printing code moved from Complex to ComplexValue
• 🐧 disable LTO in case clang is used under linux as some versions of the linker under Ubuntu do not work with Clang's ThinLTO

[codecov]

Codecov Report

Merging #10 (a1b81f3) into master (6fc1e37) will increase coverage by 0.0%.
The diff coverage is 95.5%.

@@          Coverage Diff           @@
##           master     #10   +/-   ##
======================================
  Coverage    96.1%   96.1%           
======================================
  Files          16      17    +1     
  Lines        2220    2262   +42     
======================================
+ Hits         2134    2175   +41     
- Misses         86      87    +1     

Impacted Files	Coverage Δ
include/dd/Edge.hpp	100.0% <ø> (ø)
include/dd/Export.hpp	98.6% <50.0%> (-0.4%)	⬇️
include/dd/ComplexCache.hpp	97.8% <87.5%> (-2.2%)	⬇️
include/dd/UniqueTable.hpp	97.2% <90.9%> (-2.8%)	⬇️
include/dd/ComplexTable.hpp	96.0% <94.7%> (-0.1%)	⬇️
include/dd/Package.hpp	92.5% <95.6%> (+0.8%)	⬆️
include/dd/Complex.hpp	100.0% <100.0%> (+2.3%)	⬆️
include/dd/ComplexNumbers.hpp	100.0% <100.0%> (ø)
include/dd/ComplexValue.hpp	97.0% <100.0%> (-0.2%)	⬇️
include/dd/ComputeTable.hpp	90.4% <100.0%> (-5.2%)	⬇️
... and 15 more

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