Merge rust-bitcoin#5059: primitives: replace opcodes.rs with a skeleton file with bare minimum functionality

e05df3f primitives: replace opcodes.rs with a skeleton file with bare minimum functionality (Andrew Poelstra)

Pull request description:

  The `Opcode` type in primitives/src/opcodes.rs is used in two places: in primitives/script/mod.rs to parse numbers (by looking at OP_PUSHDATAx) and in primitives/script/borrowed.rs to display opcodes when outputting script "asm".
  
  We don't need the classification, we don't need the alternate names, we don't need the decoding methods, we don't need serde, we don't even need the type. We just need four constants and a formatting method.


ACKs for top commit:
  tcharding:
    ACK e05df3f


Tree-SHA512: 12c9a8fca5e264aa33649774a53157807692ce5ad549b00e22ad417dbb48afd93325b9d6bdf4b8360435453fa0dbc04b8841e6729891b403d50b9418610432fe

Author: apoelstra