add module root node in Hugr::new

[ss2165]

HUGR should have one root node always - might as well create this with new

[acl-cqc]

I was thinking the same idea, so this looks good to me. Not sure why CI is failing - IIUC the same "errors" show up as warnings when I build branch main.

[aborgna-q]

The CI is pretty pedantic rn.
The links from the warning are broken because the types have not been imported in the module. They need an explicit link as in

hugr/src/resource/mod.rs

Lines 4 to 6 in 067e4c5

	//! system (outside the `types` module), which also parses nested [`OpDef`]s.
	//!
	//! [`OpDef`]: crate::ops::custom::OpDef

Edit: The error was already happening in main, but Github Actions was down when that change was pushed. I put a fix for the CI error in #3.

[aborgna-q]

I made roots optional to allow for partial hugrs meant for rewriting. I'm not yet up to date with the current definition for that.

[acl-cqc]

I made roots optional to allow for partial hugrs meant for rewriting. I'm not yet up to date with the current definition for that.

Replacements are a forest of nodes. Whilst they could all be stored in a single HUGR I'm not sure how much sense that makes, they could just be a list (there is a ParentsMap as well, right).

[aborgna-q]

Wait for #4 before merging this.