Add TUniform distribution

[bencrts]

Adding the TUniform distribution which is used in the tfhe-rs library (as well as other FHE schemes, such as BGV/CKKS). This is essentially a generalisation (consider b=0) of the distribution which samples:

-1 with prob 1/4
1 with prob 1/4
0 with prob 1/2

which is commonly mixed up with the uniform ternary distribution. More generically, the distribution TUniform(-2**b, 2**b) samples:

-2**b, 2**b each with probability 1/2**(b+2)
the other terms with probability 1/2**(b+1)

For small values of b, the associated standard deviation is different enough from the Uniform() distribution that it is worth including both.

[malb]

lgtm