expose DocumentableItemSignatureData

[FroyaTheHen]

partially closes software-mansion/scarb#2840

This is a Scarb task but it turned out that LS also faces the problem where in some cases the custom logic needs to implemented for building an item signature.
This PR aims at:

• exposing some compiler logic so the same methods can be used by Scarb and LS,
• brings some minor redability improvements

[reviewable-StarkWare]

This change is 

[orizi]

@orizi reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @Draggu and @FroyaTheHen)

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crates/cairo-lang-doc/src/signature_data.rs line 35 at r1 (raw file):

/// A helper struct gathering documentable item's signature data.
pub struct DocumentableItemSignatureData<'db> {
    pub item_id: DocumentableItemId<'db>,

doc all field now, since pub.

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crates/cairo-lang-doc/src/signature_data.rs line 88 at r1 (raw file):

implement_signature_data_retriever!(ImplConstantDefId<'db>, get_impl_constant_signature_data);

/// A utility function retrieves [`ModuleItemInfo`] for [`ModuleItemId`].

*that retrieves

[FroyaTheHen]

Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @Draggu and @orizi)

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crates/cairo-lang-doc/src/signature_data.rs line 35 at r1 (raw file):

Previously, orizi wrote…

doc all field now, since pub.

Done.

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crates/cairo-lang-doc/src/signature_data.rs line 88 at r1 (raw file):

Previously, orizi wrote…

*that retrieves

Done.

[orizi]

@orizi reviewed 1 of 1 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @Draggu)