Move numbering systems field over to DecimalSymbolsStrs

[Manishearth]

Depends on #5822

Minor win:

Before: decimal/symbols@2, <total>, 2740B, 1368B, 49 unique payloads
After:  decimal/symbols@2, <total>, 2657B, 1285B, 49 unique payloads

And an 8 byte win on FDF stack size.

[sffc]

The stack size win is my main motivation

[Manishearth]

@sffc Thought: do you think we can get away with Index8 for DecimalSymbolStrs? Each one of its entries is at worst 3 characters (when it contains bidi format characters), and then there's the ascii numbering system, at worst 8 bytes.

(8 numsys bytes) + (3 chars) * (2-4 bytes per char) * (6 fields) is 44-80, which is still well in range for a u8.

[sffc]

Yes we should definitely use Index8

[dpulls]

🎉 All dependencies have been resolved !

[Manishearth]

Before: decimal/symbols@2, <total>, 2657B, 1285B, 49 unique payloads
After:  decimal/symbols@2, <total>, 2363B, 991B, 49 unique payloads

For Index8.