[spec] Many definitions under "Entropy Budgets" section needs improving

[xyaoinum]

Methods can't really have state attached to them. It's better to make a struct.

• calling site: https://github.com/WICG/shared-storage/pull/131/files#r1563108312, https://github.com/WICG/shared-storage/pull/131/files#r1563109589
• input URL list. https://github.com/WICG/shared-storage/pull/131/files#r1563119786
• default index: https://github.com/WICG/shared-storage/pull/131/files#r1563120997