Add `ctx.rng_{weighted|ratio}_bool()` helper functions

[abey79]

This PR adds to functions to whiskers::Context:

• ctx.rng_weighted_bool(prob: f64): random bool with give probability of true
• ctx.rng_ratio_bool(num: u32, denim: u32): same, but with probability expressed as a num/denom ratio

The latter variant is useful for when dealing with, e.g., column or row index:

let some_bool = ctx.rng_ratio_bool(current_columns, (total_column - 1));