Add ⊗ as synonym for kron
Add √ as synonym for sqrt
Add obelus (÷) as synonym for div
Add ∅ (empty set) as synonym for Set{Any}({})
Add ∈ as synonym for `in` and ∋ for containment
Add ⋅ as synonym for `dot`
Add ⊆ as synonym for `issubset`
More set relations ⊈, ⊂, ⊄, ⊊.

- Strict subsets ⊂, ⊊
- Negated synonyms ⊈, ⊄
Add ∩ and ∪ as synonyms for `intersect` and `union` resp.
Add △ as synonym for `symdiff`
Add negated element containments ∉, ∌
More comparsion operators ≠ (!=), ≡ (===), ≢ (!==), ≤ (<=), ≥ (>=), .≥ (.>=), .≤ (<=), .≠ (.!=)
Add double solidus (⫽) as synonym for rational division (//)

Is (.//) defined anywhere?
Adds ∣ (divides) and its negation ∤
Add × (cross product) as synonym for `cross`
Remove ∅ (empty set) and undefines ⊂ and ⊄

⊂ and ⊄ seem useful to have around for user-defined relations, but otoh it seems odd to define infix operators that are never used in Base.
Changed operator precedence for ∩ and ∪. Undefine △.
Add ∨ and ∧, and circled and boxed arithmetic operators

⊕ ⊖ ⊗ ⊘ ⊙ ⊚ ⊛ ⦸ ⦼
⊞ ⊟ ⊠ ⊡
Skip PCRE tests on Windows

These constantly fail for silly reasons like line endings or
not having the French locale installed.

May as well just save a manual step and needing to refer people
to an easily-missed item in README.windows.md.
Closes #6631