Remove unsafe without sacrificing speed by replacing i64 "exponent" with primitive enum

[Protowalker]

By replacing the exponent field in the Number struct with two fields, one for an enum representing the different possible values and another for if the exponent is negative, you can remove the unsafe in float.rs:

unsafe { *TABLE.get_unchecked(exponent)}

can be replaced with

TABLE[exponent as usize]

with little to no speed change over the unsafe counterpart. In my (quite limited) testing, the unsafe code actually performed slower because of the dereference.

[Protowalker]

To clarify, this enum would look like this:

enum Exponent {
    e0 = 0,
    e1 = 1,
    e2 = 2,
    e3 = 3,
    e4 = 4,
    e5 = 5,
    e6 = 6,
    e7 = 7,
    e8 = 8,
    e9 = 9,
    e10 = 10,
    e11 = 11,
}

[Protowalker]

Alternative approach that I haven't tested for speed but should compile into a fast lookup table:

match num {
   0 => 1,
   1 => 10,
   2 => 100,
   3 => 1000,
   ...
   _ => unreachable!() 
}

EDIT: this approach is too slow