implement fee burning mechanism

[brenzi]

Along the tokenomics paper Sec. 6:

We need to

• count the cumulative number of extrinsics on the chain
• upon fee payment, burn a fraction of the fees according to the formula in the paper
  • a lookup table with 100 entries is sufficient: yields burning rate resolution of 1%. that's smooth enough.
  • only if more efficient, use fixpoint math instead.
• transfer the remainder to the treasury

learn more about burning and imbalances: https://blog.polymath.network/substrate-deep-dive-imbalances-8dfa89cc1d1?gi=7948e9237106

[brenzi]

first step is done in: #43

• burn a constant fraction (99%), rest to treasury

[brenzi]

counting extrinsics per block is already done here:
https://github.com/paritytech/substrate/blob/ba153b9ae050eda022f002d74d76f98d1e339a81/frame/system/src/lib.rs#L582
however, AFAIK there is no global monotonic counter yet

Option 1

implement a dedicated pallet that just features on_finalize and adds ExtrinsicCount to the global counter it holds