Messages return TypeSpec directly

[SkymanOne]

Summary

Closes #1515

• [n] y/n | Does it introduce breaking changes?
• [n] y/n | Is it dependant on the specific version of cargo-contract or pallet-contracts?

Just a minor code refactoring.
Similarly to constructors, messages return TypeSpec directly due to the introduction of LangError

Checklist before requesting a review

• My code follows the style guidelines of this project
• I have added an entry to CHANGELOG.md
• I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works
• Any dependent changes have been merged and published in downstream modules

[codecov-commenter]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (a9401be) 53.24% compared to head (178d4ff) 53.27%.

Files	Patch %	Lines
crates/ink/codegen/src/generator/metadata.rs	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1999      +/-   ##
==========================================
+ Coverage   53.24%   53.27%   +0.03%     
==========================================
  Files         220      220              
  Lines        6846     6845       -1     
==========================================
+ Hits         3645     3647       +2     
+ Misses       3201     3198       -3     

☔ View full report in Codecov by Sentry.
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[github-actions]

🦑 📈 ink! Example Contracts ‒ Changes Report 📉 🦑

These are the results when building the integration-tests/* contracts from this branch with cargo-contract and comparing them to ink! master:

Contract	Upstream Size (kB)	PR Size (kB)	Diff (kB)	Diff (%)	Change
basic-contract-caller	2.967	2.967	0	0	➖
basic-contract-caller/other-contract	1.337	1.337	0	0	➖
call-builder-return-value	8.735	8.735	0	0	➖
call-runtime	1.769	1.769	0	0	➖
conditional-compilation	1.209	1.209	0	0	➖
contract-storage	7.171	7.171	0	0	➖
contract-terminate	1.092	1.092	0	0	➖
contract-transfer	1.444	1.444	0	0	➖
custom-allocator	7.428	7.428	0	0	➖
dns	7.249	7.249	0	0	➖
e2e-call-runtime	1.058	1.058	0	0	➖
e2e-runtime-only-backend	1.635	1.635	0	0	➖
erc1155	13.962	13.962	0	0	➖
erc20	6.687	6.687	0	0	➖
erc721	9.64	9.64	0	0	➖
events	4.763	4.763	0	0	➖
flipper	1.393	1.393	0	0	➖
incrementer	1.221	1.221	0	0	➖
lang-err-integration-tests/call-builder-delegate	2.317	2.317	0	0	➖
lang-err-integration-tests/call-builder	4.847	4.847	0	0	➖
lang-err-integration-tests/constructors-return-value	1.773	1.773	0	0	➖
lang-err-integration-tests/contract-ref	4.328	4.328	0	0	➖
lang-err-integration-tests/integration-flipper	1.571	1.571	0	0	➖
mapping-integration-tests	7.685	7.685	0	0	➖
mother	9.508	9.508	0	0	➖
multi-contract-caller	5.924	5.924	0	0	➖
multi-contract-caller/accumulator	1.095	1.095	0	0	➖
multi-contract-caller/adder	1.669	1.669	0	0	➖
multi-contract-caller/subber	1.689	1.689	0	0	➖
multisig	21.471	21.471	0	0	➖
payment-channel	5.501	5.501	0	0	➖
sr25519-verification	0.865	0.865	0	0	➖
static-buffer	1.405	1.405	0	0	➖
trait-dyn-cross-contract-calls	2.466	2.466	0	0	➖
trait-dyn-cross-contract-calls/contracts/incrementer	1.305	1.305	0	0	➖
trait-erc20	7.063	7.063	0	0	➖
trait-flipper	1.209	1.209	0	0	➖
trait-incrementer	1.37	1.37	0	0	➖
upgradeable-contracts/delegator	2.908	2.908	0	0	➖
upgradeable-contracts/delegator/delegatee	1.369	1.369	0	0	➖
upgradeable-contracts/set-code-hash	1.464	1.464	0	0	➖
upgradeable-contracts/set-code-hash/updated-incrementer	1.443	1.443	0	0	➖
wildcard-selector	2.622	2.622	0	0	➖

Link to the run | Last update: Mon Nov 20 20:42:32 CET 2023