Problem 951. Unique: Faster for options 'rows' and 'stable' for large array of uint8

Challenge: Execute unique(a,'rows','stable') Faster for 'a' being uint8.

The "unique" function for the 'rows' and 'stable' options may be performed 10X more quickly for arrays of large rows, few columns, and uint8 (or values <256).

Input: c=randi(4,6000000,16,'uint8')-1;

Output: unique(c,'rows','stable') or equivalent

Scoring: Time in milli-seconds

Hints:

• unique can sort a single column >10X faster than an array of rows
• 10X improvement is applicable to arrays where rows can be represented as a uint32 value
• 5X improvement occurs where arrays require uint64 representation