feat: experimental acir optimisation

[guipublic]

Description

Problem*

Related to #6085

Summary*

This PR is an attempt to perform the 'as_witness' automatically.
The idea is to create witness for each common expression, instead of having the user to specify the expressions to fit into a witness. Because this strategy generates a lot of 'small' arithmetic opcodes, I benefit from the big-add-gate (cf. #6085) by simplifying those small opcodes into the ones which use them.

Additional Context

In general this optimisation works well, but some corner cases still need to be investigated. Indeed, the simplification may generate too many opcodes which do not fit the width.
Hence, it is hidden by a non-documented compiler option (experimental-optimisation).
I leave it as a draft for now.

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.