Refactor spec/version matching.

Instead of choosing the comparison on each `.match()` call, the
expression is converted to a combination of `Range()` expressions
(simple comparison to a semver-compliant `Version`).

`Range()` objects can be combined with `And` and `Or` through the
`AnyOf` and `AllOf` clauses (sticking to Python's naming scheme).

Some specific flags have been provided to those range, allowing users to
subtly alter the matching behaviour - thus accomodating different
versioning schemes:
- `<0.1.2` won't match `0.1.2-rc1`, unless the prerelease_policy flag is
  set to either `always` or `same-patch`
- `<0.1.2` will match `0.1.1-rc1`, unless the `prerelease_policy` flag
  is set to `same-patch`
- `==0.1.2` will always match `0.1.2+build44`, unless the `build_policy`
  is set to `strict`.

The `Spec` item has been updated, alongside `SpecItem`.
Those objects keep the original expression as attributes, but don't use
them for comparisons.

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