Potential unsoundnesses (not yet determined) with use of unsafe

[Alexhuszagh]

From the upstream fast-float-rust:
aldanor#37

[Alexhuszagh]

There's a few core locations where this is unsafe with debug-only invariant checks. The most obvious location is:

fast-float-rust/src/number.rs

Line 151 in cdcde1e

debug_assert!(!s.is_empty());

Where we've checked that s is not empty in the caller, but this does not have local safety invariants and therefore is not a safe function.