`rerandomize_global_context` is only called if seed is passed in

[tcharding]

rerandomize_global_context does not use the seed if rand feature is enabled however the function is only called if a seed is passed in. For rand users it would be more ergonomic to call it unconditionally so they can use None.

Also the docs are not exactly correct IIUC

/// Rerandomize the global context, using the given data as a seed.
///
/// The provided data will be mixed with the entropy from previous calls in a timing
/// analysis resistant way. It is safe to directly pass secret data to this function.
pub fn rerandomize_global_context(seed: &[u8; 32]) {

[tcharding]

Perhaps to #847 at the same time.

[apoelstra]

@tcharding it's expensive to call the rerandomize function. Users should have a way to access the context without calling it. I used an Option to achieve that.

We can split it into two functions if you think that'd be a better API.

[tcharding]

Ah I see, that was not at all clear to me. Let me take a look and see if I can make it more clear ether through the API or some docs. Thanks man

[tcharding]

Let's just leave it as is.