Refactor and modernize dispatch codegen

[Robbepop]

Work carved out of #665.

• Improves macro hygiene of dispatch codegen.
• Modernizes generated selector namespaces of ink! messages and ink! constructors.
  • Uses const generic parameters instead of abusing array lengths.
• Improves panic messages for debugging dispatching.
• Uses big-letter hex-encoded IDs for generated message and constructor info types.

[codecov-commenter]

Codecov Report

Merging #933 (8bb7d68) into master (41b0a1c) will increase coverage by 2.95%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master     #933      +/-   ##
==========================================
+ Coverage   84.20%   87.15%   +2.95%     
==========================================
  Files         173      169       -4     
  Lines        7939     7561     -378     
==========================================
- Hits         6685     6590      -95     
+ Misses       1254      971     -283     

Impacted Files	Coverage Δ
crates/lang/ir/src/ir/selector.rs	75.00% <66.66%> (-10.72%)	⬇️
crates/lang/codegen/src/generator/dispatch.rs	99.42% <90.90%> (-0.58%)	⬇️
crates/env/src/engine/off_chain/db/chain_spec.rs	79.54% <0.00%> (-20.46%)	⬇️
crates/env/src/engine/off_chain/chain_extension.rs	57.14% <0.00%> (-7.86%)	⬇️
crates/env/src/engine/off_chain/db/events.rs	93.54% <0.00%> (-6.46%)	⬇️
crates/env/src/engine/off_chain/db/accounts.rs	81.15% <0.00%> (-5.43%)	⬇️
crates/env/src/engine/off_chain/mod.rs	86.95% <0.00%> (-2.66%)	⬇️
crates/metadata/src/lib.rs	0.00% <0.00%> (ø)
crates/primitives/src/key_ptr.rs	100.00% <0.00%> (ø)
crates/env/src/backend.rs
... and 33 more

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