checker improvement: make floats checker use arbitrary-precision Decimal

[xiaowuc1]

Most floating-point checkers do not depend on this, however it can avoid issues when the precision guarantee needed is very tight.

[Xyene]

I feel like the "fast path" for this (10e-6?) should use regular floats for speed, and switch to Decimal when needed.

[xiaowuc1]

Adding a flag that defaults to fast and uses floats in that case, and the Decimal otherwise seems like a reasonable approach.