Make Option.to_seq() return a Seq and add .of classmethods to Seq

[erlendvollset]

Option.to_seq() -> Seq requires some if TYPE_CHECKING and deferred import hacks to avoid circular dependencies. What do you think?

[codecov[bot]]

Codecov Report

Merging #9 (0050c20) into main (0319f5d) will increase coverage by 0.00%. The diff coverage is 84.21%.

@@           Coverage Diff           @@
##             main       #9   +/-   ##
=======================================
  Coverage   80.19%   80.20%           
=======================================
  Files          37       37           
  Lines        2111     2122   +11     
=======================================
+ Hits         1693     1702    +9     
- Misses        418      420    +2     

Impacted Files	Coverage Δ
expression/collections/seq.py	87.91% <83.33%> (-0.16%)	:arrow_down:
expression/core/option.py	87.36% <83.33%> (-0.21%)	:arrow_down:
expression/core/aiotools.py	62.79% <100.00%> (ø)