Created exercise for separable differential equations

Initial value is included. Only in dx/dy = M(x)/N(y) form so far. All
numbers lead to non-fractionated answers (without additional
multiplication). The data-ensure will work about 2 out of every 3 times
(based on Monte Carlo simulation). Hints are included (including
multiplication, integration, and substitution steps). Solution uses
'expression' data-type.

Note that I used workarounds to avoid the ["+","z",["*",-1,["^","x",0]]
case, which evaluated to 'z -' rather than 'z - 1' and hardcoded C
because I didn't know how to use a function (if possible) in the
'expressions' module to evaluate an expression. The same goes for the
answer. If such a function exists, this should be cleaned up for
consistency.

Also, separable should be spelled with two 'e's rather than two 'a's.
Or should I say spelt.