Change project structure

[canepat]

This PR restructures the project in order to better reflect the modular design of Silkworm and achieve the following objectives:

• all source files of the project are contained within one folder named silkworm: this makes the project layout simpler and more intuitive for newcomers
• the folders at the first level within such silkworm folder represent the modules composing the system: this helps to immediately find a mapping between the modular design and the source files
• the format of the inclusion directives becomes <silkworm/mmm/...>, where mmm is one of the system modules: this enforces the clear distinction between the internal and external includes and also allows to figure out which module the included header belongs to

[codecov]

Codecov Report

Merging #878 (be863be) into master (985f0ff) will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #878      +/-   ##
==========================================
+ Coverage   70.36%   70.38%   +0.02%     
==========================================
  Files         239      239              
  Lines       18256    18256              
  Branches     2811     2811              
==========================================
+ Hits        12845    12849       +4     
+ Misses       4267     4264       -3     
+ Partials     1144     1143       -1     

Impacted Files	Coverage Δ
silkworm/core/chain/config.cpp	91.55% <ø> (ø)
silkworm/core/chain/config.hpp	0.00% <ø> (ø)
silkworm/core/chain/dao.cpp	100.00% <ø> (ø)
silkworm/core/chain/difficulty.cpp	100.00% <ø> (ø)
silkworm/core/chain/genesis.cpp	80.35% <ø> (ø)
silkworm/core/chain/genesis_goerli.cpp	100.00% <ø> (ø)
silkworm/core/chain/genesis_mainnet.cpp	100.00% <ø> (ø)
silkworm/core/chain/genesis_rinkeby.cpp	100.00% <ø> (ø)
silkworm/core/chain/genesis_sepolia.cpp	100.00% <ø> (ø)
silkworm/core/chain/intrinsic_gas.cpp	100.00% <ø> (ø)
... and 335 more

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