[Internal, New Feature, No Breaking] Rewrite resolver for customization and user-selectable schema

[Paalon]

Rewrite Resolver. This enables to implement custom resolvers. I implemented resolutions as vectors but dicts may be better. However, it breaks the order, I leave it as it is.
There are 4 completed resolvers:

• A resolver for the failsafe schema.
• A resolver for the JSON schema.
• A resolver for the Core schema.
• A resolver for the YAML.jl v0.4.10 schema.

This PR is for #151, #152 and #170.

[GunnarFarneback]

This is beyond my level of study of the YAML spec but the code looks fine and there's nothing I find to be strange.

[kescobo]

This seems like a good way to go - another option would be a NamedTuple, since we don't need it to be mutable. But this seems fine to me.

I think everything I've merged in the last couple of days are stylistic/internal cleanup, no new features, is that right? On the one hand, this doesn't implement anything user-facing, so I don't think it actually needs a feature release. On the other hand, since this is starting down the path to a major new feature, I wonder if we should get all of the minor stuff merged, cut a new minor release, and then merge this.

Another option would be to start a new release branch so that stuff could continue to get developed there, but I don't know that I can handle that level of mental overhead. What do you think @Paalon ?

[Paalon]

another option would be a NamedTuple, since we don't need it to be mutable. But this seems fine to me.

If we want to change, it will be easy so think it later.

I think everything I've merged in the last couple of days are stylistic/internal cleanup, no new features, is that right? ...

Yes. This is also internal and has no external change but add new functions for further coming features. So we don't need feature release. (However we have fixed many bugs, it may be a good time to release a patch (v0.4.12))

Another option would be to start a new release branch so that stuff could continue to get developed there, ...

I want dev branch for publishing (for anyone can see and check and find a fault in my latest implementation) and experimenting the new features safely. I think there is still a serious bug around YAML document iterators etc., so master branch with only current features will need to be updated randomly in the future.

but I don't know that I can handle that level of mental overhead. What do you think ?

I need to prepare for my lab's reading seminar, so I am busy and might not be able to respond and develop until around 6 PM (GMT+9) on June 20th. It's a volunteer work, feel free to put your plans first. However the fact that there are less reviewers makes it difficult to decide the design and develop. Is it possible to give me a restricted write permission to a specific dev branch? Thankfully, @GunnarFarneback might review my work. In your free time, if you notice something wrong, then please point out and I fix it in dev branch. #172 and #173 and rewrite of src/YAML.jl and src/constructor.jl are needed for further schema development and debugging (and src/scanner.jl for YAML 1.2 & 1.1 compatibility).

[kescobo]

I've lost the plot - where does this stand?

[codecov]

Codecov Report

Attention: Patch coverage is 62.50000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 87.09%. Comparing base (3b2b353) to head (7aa3d0c).

Files	Patch %	Lines
src/resolver.jl	62.50%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #172      +/-   ##
==========================================
- Coverage   87.19%   87.09%   -0.11%     
==========================================
  Files          15       15              
  Lines        1664     1674      +10     
==========================================
+ Hits         1451     1458       +7     
- Misses        213      216       +3     

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