Eliminate flo:epsilon.

Eliminate flo:epsilon.

The name is confusing.  It means two different things: the distance
from 1 to the next positive floating-point number, and the maximum
relative error of rounding to the nearest floating-point number.
Provide the separate names flo:ulp-of-one and flo:error-bound for
these so that you're not even tempted to be confused by the name
`epsilon'.

(But continue to say `eps' in the proofs in the comments where it's
obvious that we're talking about relative error bounds and a short
name for that is essential for legibility.)