Fixy some real world use cases

[Totto16]

See https://github.com/Totto16/bignum-notes

• Make add and sub (mul? and div? too??) inline operations, this reduces allocations in many cases, provide functions for both!
• make the BigInt to string conversion faster, how? that is hard to say, but the 1000_000 Fibonacci digit (~200k digits in base 10) takes really long to print, gmp manages that in ~ 200 ms 😓