This is a correct but somewhat over-engineered reinplementation of `SymbolGraph.SemanticVersion`.

The most important (and also most invasive) changes are the handling of pre-release and build metadata. These include their validation and (their contribution to version-) comparison.

Some less important changes include the following:

- Version core identifiers are changed to be backed by `UInt`, because they’re not allowed be less than 0. This might appear to be a departure from the convention of using `Int` for even always-positive integers (e.g. `Array.Index`). However, in this specific case, the use of version coure identifiers is mostly contained in the domain of constructing semantic versions, and does not have any apparent need to participate in numeric operations outside of the domain other than comparing individual identifiers. And considering Swift’s current lack of build-time evaluation and error handling, using `UInt` looks like the right trade off between ergnomics and “safety”.

- Deprecated the initializer for a new one that throws error, so that erros can be handled instead of runtime trapping.

- Added some facilities for inspecting and working with versions’ semantics in general.

- Updated the tools version specifier in the main package manifest to “5.6”, because Swift 5.2 has some type-checking bugs that affect the new implementation.

- Added a lot of tests, which should cover a large area of (edge) cases.

It appears that whatever caused the type checking problems before has already been fixed.

…litting the prerelease into identifiers

Co-authored-by: David Rönnqvist <[email protected]>

…ifier` when validating build metadata

Co-authored-by: David Rönnqvist <[email protected]>

…h statements

It’s used for more than validating prerelease.

This initializer should be much more ergonomic to use in the common cases, and it's safe because it's guaranteed not to encounter any error.

Reference to [SR-14665](swiftlang/swift#57016) links to more explanation on why the manual `==` implemenatation is necessary.

…ssertions