Implement parametric trait types

[tk3369]

This PR is to implement the idea as described in #23

• [x] Code changes
• [x] Test suite update
• [x] Documentation update

An interesting artifact of switching to this design is that the commonly-used trait prefix types (Can, Is, Not, etc.) are predefined as aliases to Positive and Negative prefix types. In order to avoid polluting the namespace, they are defined in a submodule BinaryTraits.Prefix. The user just needs to import whatever they need like this:

using BinaryTraits.Prefix: Can, Cannot

Thinking further ahead, the prefix clause in @trait macro becomes a little redundant. It feels somewhat convenient though. So, I have kept the code and and make it auto-import the prefix types for now.

Cc: @KlausC @akels

[codecov[bot]]

Codecov Report

Merging #47 into master will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #47   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            7         7           
  Lines          239       241    +2     
=========================================
+ Hits           239       241    +2     

Impacted Files	Coverage Δ
src/BinaryTraits.jl	100.00% <ø> (ø)
src/misc.jl	100.00% <ø> (ø)
src/assignment.jl	100.00% <100.00%> (ø)
src/interface.jl	100.00% <100.00%> (ø)
src/trait.jl	100.00% <100.00%> (ø)
src/types.jl	100.00% <100.00%> (ø)
src/utils.jl	100.00% <100.00%> (ø)

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[tk3369]

Alright, this PR is pretty much ready to go. I'm planning to merge to master and announce to the community this weekend. Please review code if possible. Additional changes are listed as follows:

Renamed functions

While updating documentations, I realized that some functions are not consistently named. I have changed them as follows:

• istrait renamed to is_trait
• inittraits renamed to init_traits
• set_verbose renamed to set_verbose!

how do we refer to trait types?

I used to call the super-type a trait e.g. Fly would be a trait. Then, the positive and negative types were "can-type" and "cannot-type" (or "can-trait" or "cannot-trait"). Honestly, this naming scheme is quite awkward.

I realized that it's OK to just refer the positive and negative trait types as traits. So Can{Fly} is a trait and Cannot{Fly} is also a trait. Then, what do we call the abstract type Fly itself? I don't know. Suggestions welcome. For now, I have made the is_trait function to return true for all of them.