proposed changes to use more of Moo, second attempt

[dakkar]

(This PR contains the same changes as #15, but organised in a way that should make it much easier to review)

In reference to #14 :

• type constraints everywhere: among other things, this makes clearer what each attribute can accept
• defaults for all attributes where it makes sense
• BUILDARGS/new instead of common/build, this makes classes easier to use and extend
• op now canonically accepts a coderef (previously it did, but probably by mistake: it's a useful feature, let's support it properly)

Maybe needed before merging:

• documentation for all the type constraints
• better names for them

Possible future work:

• review the various enums
• review the various union types, and check if they really make sense
• reduce the number of rw attributes

[dakkar]

I suggest looking at one commit at a time, in order: I've tried to keep them self-contained (tests always pass, for example) and focused (one file, or one class of type constraints per commit, for example).