Reworked message dispatch logic. Moved common parts upper.

[xgreenx]

It is PR for the change #998. With this change Erc20 has size:
Original wasm size: 64.4K, Optimized: 28.3K

[Robbepop]

Unfortunately I cannot review this atm, however, I'd really like to review this PR before it is being merged. I will do that asap.

[cmichi]

@xgreenx I just merged #998, which automatically deleted that branch. In consequence this PR was closed by GitHub. I now restored the branch from #998, but you can change the base branch of this PR to ink! master now.

[codecov-commenter]

Codecov Report

Merging #999 (6676c7c) into master (1393502) will decrease coverage by 0.35%.
The diff coverage is 18.18%.

@@            Coverage Diff             @@
##           master     #999      +/-   ##
==========================================
- Coverage   78.78%   78.43%   -0.36%     
==========================================
  Files         244      245       +1     
  Lines        9188     9232      +44     
==========================================
+ Hits         7239     7241       +2     
- Misses       1949     1991      +42     

Impacted Files	Coverage Δ
crates/env/src/api.rs	36.36% <0.00%> (ø)
crates/env/src/backend.rs	0.00% <ø> (ø)
crates/env/src/buffer.rs	0.00% <ø> (ø)
...tes/env/src/engine/experimental_off_chain/impls.rs	55.23% <0.00%> (ø)
crates/env/src/engine/off_chain/impls.rs	42.98% <0.00%> (ø)
crates/lang/src/codegen/dispatch/execution.rs	0.00% <ø> (ø)
crates/lang/codegen/src/generator/dispatch.rs	100.00% <100.00%> (ø)
... and 2 more

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[xgreenx]

Extended version of this change in the #1017