I have most certainly heard of alchemists and while there's certainly some crossover in the witches' and alchemists' methods, they're not the same thing and the two words are not synonymous.
First recorded use of the word "alchemist" - 15th century.
First recorded use of the word "witch" - 9th century.
Ordinary, illiterate people, vox populi that is, saw alchemists as witches and warlocks. Alchemy I think started in 12th century, so basically anyone that tinkered with "uncanny" materials is seen as witch/warlock. I ofcourse, think, all supernatural stuff attributed to these people is crock of crap. Which is why it didn't bother me in Robin Hood. I am more interested in real people.
But when I look back at my earlier comment, I have to retract it and apologize for stating that were no "witches" then, that is, those who tampered with the occult. Such people definitely existed, but not as presented in Robin Hood.