fix: return trait impl method as FuncId if there's only one

[asterite]

Description

Problem

Workaround for this issue: #6987 (comment)

Summary

I'm sure a similar issue can happen in other scenarios, though I don't know what code reproduces it. We can use this workaround until we bump into a new error and the fix it in a better way.

Alternatively, we could revert #6987

Additional Context

Documentation

Check one:

• No documentation needed.
• Documentation included in this PR.
• [For Experimental Features] Documentation to be submitted in a separate PR.

PR Checklist

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[github-actions]

Compilation Report

Program	Compilation Time
sha256_regression	1.080s
regression_4709	0.794s
ram_blowup_regression	14.980s
rollup-root	3.760s
rollup-block-merge	4.390s
rollup-base-public	29.000s
rollup-base-private	11.900s
private-kernel-tail	0.985s
private-kernel-reset	10.140s
private-kernel-inner	2.120s

[github-actions]

Execution Report

Program	Execution Time
sha256_regression	0.054s
regression_4709	0.001s
ram_blowup_regression	0.573s
rollup-root	0.105s
rollup-block-merge	0.106s
rollup-base-public	1.320s
rollup-base-private	0.514s
private-kernel-tail	0.022s
private-kernel-reset	0.385s
private-kernel-inner	0.097s

[github-actions]

Compilation Memory Report

Program	Peak Memory	%
keccak256	78.55M	0%
workspace	123.86M	0%
regression_4709	422.96M	0%
ram_blowup_regression	1.58G	0%
rollup-base-public	2.55G	0%
rollup-base-private	1.24G	0%
private-kernel-tail	200.40M	19%
private-kernel-reset	728.14M	333%
private-kernel-inner	290.46M	72%

[github-actions]

Execution Memory Report

Program	Peak Memory	%
keccak256	74.65M	0%
workspace	123.91M	0%
regression_4709	315.98M	0%
ram_blowup_regression	512.41M	0%
rollup-base-public	492.17M	0%
rollup-base-private	332.25M	0%
private-kernel-tail	181.08M	7%
private-kernel-reset	254.94M	51%
private-kernel-inner	212.77M	26%