Change twice used large const table to static

This table is used twice in core::num::dec2flt::algorithm::power_of_ten.
According to the semantics of const, a separate huge definition of the
table is inlined at both places.

    fn power_of_ten(e: i16) -> Fp {
        assert!(e >= table::MIN_E);
        let i = e - table::MIN_E;
        let sig = table::POWERS.0[i as usize];
        let exp = table::POWERS.1[i as usize];
        Fp { f: sig, e: exp }
    }

Theoretically this gets cleaned up by optimization passes, but in
practice I am experiencing a miscompile from LTO on this code. Making
the table a static, which would only be defined a single time and not
require attention from LTO, eliminates the miscompile and seems
semantically more appropriate anyway. A separate bug report on the LTO
bug is forthcoming.

Author: dtolnay

core/src/num/dec2flt/table.rs

@@ -5,7 +5,7 @@ pub const MIN_E: i16 = -305;
 pub const MAX_E: i16 = 305;
 
 #[rustfmt::skip]
-pub const POWERS: ([u64; 611], [i16; 611]) = (
+pub static POWERS: ([u64; 611], [i16; 611]) = (
     [
         0xe0b62e2929aba83c,
         0x8c71dcd9ba0b4926,