Make renf_elem_randtest generate small elements

[videlec]

As mentioned in #142 it is desirable that renf_elem_randtest generate nonzero elements with very small embedding.

[slel]

Tentative approach:

• take the output of the current random element generator
• if nonzero, raise it to a random integer power (positive or negative)

Assuming the current random element generator usually gives numbers
with absolute value above 2 or below 1/2, and assuming the random integer
picked as the exponent is sometimes large, this should provide very small
numbers with some frequency.

[videlec]

No you want to use LLL here. (in order to keep small coefficients)

[saraedum]

I would also like to see 0, some rational numbers and numbers whose embedding is infinite.

Making something like 0 show up here is tricky without having it show up too often, so maybe I want to make this the first value that always comes out of the RenfElemGenerator.