Merge rust-bitcoin/rust-bitcoin#716: Add amount::Display - make formatting configurable

4f1200d Added `amount::Display` - configurable formatting (Martin Habovstiak)

Pull request description:

  This significatnly refactors the formatting code to make formatting more
  configurable. The main addition is the `Display` type which is a
  builder that can configure denomination or other things (possibly more
  in the future).

  Further, this makes all representations of numbers minimal by default,
  so should be documented as a possibly-breaking change.

  Because of the effort to support all other `fmt::Formatter` options this
  required practically complete rewrite of `fmt_satoshi_in`. As a
  byproduct I took the opportunity of removing one allocation from there.

  Closes #709

ACKs for top commit:
  tcharding:
    ACK 4f1200d
  dr-orlovsky:
    ACK 4f1200d
  sanket1729:
    ACK 4f1200d

Tree-SHA512: 3fafdf63fd720fd4514e026e9d323ac45dfcd3d3a53a4943178b1e84e4cf7603cb6235ecd3989d46c4ae29453c4b0bb2f2a5996fbddf341cd3f68dc286062144

Author: ChallengeDev210