Make bernoulli numbers with mpreal work

[emsr]

Trying to make bernoulli numbers with mpreal was a sobering exercise!
The sums in my code were better! You need to cast ints to mpreal or they go to double!
I've patched my table generators but the issue is deep.
num_thing_t may need to be a functions (constexpr, trivial for basic types)
that for mpreal stores the input precision. It is a cast operator.

I should solicit ideas for numeric_limits for precision-erased number types.