Put an `UnsafeCell` on the `static mut` context

[apoelstra]

It is probably UB to assign to this object without an unsafecell around it. Unless we are going to remove it entirely before the next release, we should stick an UnsafeCell around it.

Even if we do drop this before the next release, if we have a small enough patch we might wanna backport it. Though there haven't been any user reports of visible problems.

May also be worth filing a clippy issue to lint any direct assignments to Drop static muts.

Based on post by @Kixunil in #794 (comment)