Rename first -> lowerbound, last -> upperbound

[haberdashPI]

This is an experiment to stop overloading first and last from Base, as per #90, since it is very pun-like.

I've tried to strike a balance that makes this minimally disruptive to users of the package, while also making the internal vocabulary used by the package not confusing.

I've renamed these two types

• LeftEndpoint -> LowerEndpoint
• RightEndpoint -> UpperEndpoint

Leaving the old names as aliases to avoid breaking old code.

I've also renamed the Direction types to Lower and Upper.

Finally, I've marked this as 2.0 and removed any old deprecations, and added in new ones for first and last as well as the constructors for LeftEndpoint and RightEndpoint.

I think this this would be an improvement in nomenclature, but even with my efforts to avoid disruption, it will still invovle churn.

What do folks think? Is this worth it?

[codecov]

Codecov Report

Merging #198 (e448117) into master (a785b56) will increase coverage by 8.85%.
The diff coverage is 95.47%.

@@            Coverage Diff             @@
##           master     #198      +/-   ##
==========================================
+ Coverage   84.16%   93.02%   +8.85%     
==========================================
  Files          12        9       -3     
  Lines         840      688     -152     
==========================================
- Hits          707      640      -67     
+ Misses        133       48      -85     

Impacted Files	Coverage Δ
src/plotting.jl	94.11% <75.00%> (ø)
src/interval_sets.jl	85.41% <93.87%> (-6.50%)	⬇️
src/anchoredinterval.jl	97.45% <95.65%> (-1.67%)	⬇️
src/interval.jl	96.01% <95.83%> (ø)
src/Intervals.jl	100.00% <100.00%> (ø)
src/description.jl	100.00% <100.00%> (ø)
src/endpoint.jl	98.11% <100.00%> (ø)
src/parse.jl	100.00% <100.00%> (ø)

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