Fix #2901: VersionRange.toString() doesn't handle >0.0.0 ...

The `VersionRange.toString` method was not ideal and dropping
this rare case. A tiny improvement that was also done in the
process is to give higher priority to `matchesAny` so that
we get a more user-friendly output, given how common it is
(e.g. for `ToolchainRequirement`).

Add some missing fields to the SDL<->JSON conversion test.

Avoid AA order reliance of test.

Add test for "dub convert".