Function frequencyByGroup

Computes the frequency of values by a grouping function.

const data = [1,2,3,4,5,6,7,8,9,10];
// Returns 'odd' or 'even' for an input value

const groupBy = v => v % 2 === 0 ? `even`:`odd`;

const data = frequencyByGroup(groupBy, data);
// Yields map with:
//  key: 'even', value: 5
//  key: 'odd', value: 5
@param groupBy 
@param data 
@
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