Update sanctuary-type-identifiers to version 3

[Avaq]

This update has been a long time coming. I'll explain the reason for waiting so long below, alongside the relevant diff. This update is possibly a breaking change, but I don't see yet how. Maybe @davidchambers can think of a scenario where it would break someone's code. :)

Closes #418 by supersession.

[codecov]

Codecov Report

Merging #425 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #425   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           46        46           
  Lines         1976      1987   +11     
=========================================
+ Hits          1976      1987   +11     

Impacted Files	Coverage Δ
src/future.js	100.00% <100.00%> (ø)
src/par.js	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 83b0894...87a3eda. Read the comment docs.

[Avaq]

I just realized how this is a breaking change: The removed Future['@@type'] property was used as a public interface by some future-* libs.

[Avaq]

This PR is no longer breaking because @davidchambers came up with a way to preserve compliance with sanctuary-type-identifiers versions 1 and 2 without interfering with sanctuary-type-identifiers 3: d3c2a25#diff-f9b600f4388a4ec2c76b0b6648a13b44R62-R67

[davidchambers]

$$type is not of type Type but of type String. 🤪

I think $$type should be replaced with Future a b in the comment above.

The comment could begin with To prevent rather than In order to prevent without any loss of meaning.

[Avaq]

I don't like bringing the type parameters into the picture, because they have nothing to do with sanctuary-type-identifers. I rephrased that part though. What do you think of the current phrasing?