Switch to native int/num decode methods.

[bbkr]

Rakudo now has built-in methods that duplicate custom decodings in BSON code:
https://docs.perl6.org/type/Blob#Methods_on_blob8_only_(6.d,_2018.12_and_later)

They are really fast, so maybe it's worth switching to those methods?
Current methods can be left as fallback until most Rakudo builds (Star, linux, macOS) are at least 2019.01 (there were some issues with endian flags fixed after 2018.12).

[MARTIMM]

I am so sorry that I haven't seen your input. I somehow had unwatched the repo and therefore did not notice.
I'll will look into it

[MARTIMM]

I've implemented it. It's a pity it only provides reading (decoding) and not writing. All in all, my benchmark of a mixed elements doc is not really much faster though. About 45 encode/decode ops in a second. However, much better than the first test I ever did, about 6 times/sec. Most of the improvement is accounted for by Raku I think.

[MARTIMM]

Oops, found the 'write' ones too! Still not much better. 46/47 times/sec.