Follow-up: Implement storage2 trait derive proc. macros

[Robbepop]

Follow up for #311 .
Adds derive proc. macros for the SpreadLayout and PackedLayout traits of the new ink_core::storage2 module.

Must not be merged before #311 is merged.

This PR also improves the structure of the ink_core/derive crate.
Moves AllocateUsing and Flush derives into their own files.
Move to synstructure tests for the new trait derive proc. macros instead of the current test framework.

[codecov-commenter]

Codecov Report

Merging #399 into master will decrease coverage by 0.07%.
The diff coverage is 99.00%.

@@            Coverage Diff             @@
##           master     #399      +/-   ##
==========================================
- Coverage   86.34%   86.27%   -0.08%     
==========================================
  Files         131      134       +3     
  Lines        5727     5821      +94     
==========================================
+ Hits         4945     5022      +77     
- Misses        782      799      +17     

Impacted Files	Coverage Δ
core/derive/src/spread_layout.rs	98.52% <98.52%> (ø)
core/derive/src/allocate_using.rs	100.00% <100.00%> (ø)
core/derive/src/flush.rs	100.00% <100.00%> (ø)
core/derive/src/packed_layout.rs	100.00% <100.00%> (ø)
core/src/storage2/traits/impls/tuples.rs	47.36% <0.00%> (-52.64%)	⬇️
core/src/storage2/traits/impls/prims.rs	0.00% <0.00%> (ø)

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[ascjones]

LGTM

[ascjones]

TIL