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

⊕ ⊖ ⊗ ⊘ ⊙ ⊚ ⊛ ⦸ ⦼
⊞ ⊟ ⊠ ⊡
WIP: new approach to efficiently hashing 1, 1.0, big(1), the same.

The major change is that numeric values that are mathematically
equal hash the same. This is tricky to do in a way that allows the
hashing of Int64, Float64, Uint64 and such common numeric types to
all be fast – nearly as fast as just applying the core hash to them
as raw bits.

Although tests pass, this is an inherently half-baked state since
isequal and hash are now badly out of sync. Many decisions still
need to be made about how to hash collections: does the collection
type matter or just the element type of the collection? Or neither?

This also does away with the bitmix function, instead favoring a
Merkle-Damgård style of combining arbitrary amounts of data into a
single, fixed-size hash value output. In a sense the hash function
with two arguments replaces the bitmix function – you can give the
result of hashing previous values as the second argument to hash
and the result will depend on both in a difficult-to-predict way.
decompose: fix documentation of decompose for rational hashing.