primitives: replace opcodes.rs with a skeleton file with bare minimum functionality

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.

Author: apoelstra