Draft: fix(groupBy) do not return wrapped in Partial<>

[Harris-Miller]

A previous MR #45 added to the return of groupBy that it should be wrapped in Partial<>. This makes a lot of sense in the fact that the function could return keys that don't actually exist on the object it creates.

Using the example from Ramda docs

const byGrade = groupBy((student: { score: number; name: string }) => {
  const score = student.score;
  return score < 65 ? 'F' : score < 70 ? 'D' : score < 80 ? 'C' : score < 90 ? 'B' : 'A';
});
const students = [
  { name: 'Abby', score: 84 },
  { name: 'Eddy', score: 58 },
  { name: 'Jack', score: 69 }
];

The type that byGrade(students) returned previously was Record<'A' | 'B' | 'C' | 'D' | 'F', Student[]>, which was accurate, but also misleading, because there was no guarantee at runtime that each of the union values existed as a key. This is why the Partial<> as added.

6 months later

HIndsight is 20/20 and we've come to learn that wrapping in Partial<> breaks other functionality in terms of mapping to other things. toPairs is a prime example.

const entries = toPairs(groupBy(students));

The return type previously was [<'A' | 'B' | 'C' | 'D' | 'F', Student[] | undefined][]. The | undefined is a problem. Because that it more inaccurate for runtime behavior that index accessing on Record<string, Student[]>

Typescript also allows for what we were trying to solve here via the tsconfig option https://www.typescriptlang.org/tsconfig#noUncheckedIndexedAccess. Which if set gives a project a correct Students[] | undefined from index accessing Record<string, Students[]>. Which IMHO should be the default behavior for Record<>

This MR removes the Partial<> wrap in the return to account for this undesired behavior.

[Harris-Miller]

Closing per #91 (comment)