feat: Add Type.Assert HKT that implements behavior of TypeScript as assertions, and add Type.Cast improvements

[MajorLift]

• Adds new HKT Type.Assert, which performs type casting to related subtypes and supertypes, but returns never for impossible coercions.
• Type._$cast is now non-distributive for union input params.
  • No functional change, but makes the intent of the implementation a bit clearer.
  • Rough proof of equivalence:

1. CastDistributive<T, U> = T extends U ? T : U
CastDistributive<A | B, U>
<=> | (A extends U ? A : U)
    | (B extends U ? B : U)

if (A extends U && B extends U)
  <=> A | B

if (A extends U && B not extends U)
  <=> A | U
  <=> U

if (A not extends U && B not extends U)
  <=> U | U
  <=> U

2. CastNonDistributive<T, U> = [T] extends [U] ? T : U
CastNonDistributive<A | B, U>
<=> [A | B] extends [U] ? A | B : U

if (A extends U && B extends U)
  <=> A | B

if (A extends U && B not extends U)
  => A | B not extends U
  ∴ U

if (A not extends U && B not extends U)
  => A | B not extends U
  ∴ U

[poteat]

Sensible update IMO - although I wasn't fully convinced that T extends U ? T : U is equivalent to [T] extends [U] ? T : U - to my knowledge there existed a pair ($T$, $U$) such that the equivalence did not hold true - but I wasn't able to prove this, and all the tests pass.

[MajorLift]

Maybe you were thinking of T extends never vs. [T] extends [never]? It's true that those are not equivalent statements, but T extends never ? T : never and [T] extends [never] ? T : never should both always evaluate to never.