add InterchangeEntry pretty printer

[mhuesch]

this doesn't need to be a PR but I'm creating one because I want to document how the pretty printer looks when running in rlp:

 press q to exit, Tab to switch to creator.
 ┌viewer────────────────────────────────────────────────────────────────────────────────────────┐
 │InterchangeEntry {                                                                            │
 │  operator: 1,                                                                                │
 │  operands: [ ],                                                                              │
 │  output_scheme: Int,                                                                         │
 │  output_flat_value: 1,                                                                       │
 │  start_gas: 0,                                                                               │
 │}                                                                                             │
 │                                                                                              │
 │InterchangeEntry {                                                                            │
 │  operator: +,                                                                                │
 │  operands: [                                                                                 │
 │    InterchangeOperand(uhCkkboI9VZ_V1s5pM0j97vF66wcThve2QRs_n9VlxsUk9o_SgYkg),                │
 │    InterchangeOperand(uhCkkboI9VZ_V1s5pM0j97vF66wcThve2QRs_n9VlxsUk9o_SgYkg),                │
 │   ],                                                                                         │
 │  output_scheme: Int,                                                                         │
 │  output_flat_value: 2,                                                                       │
 │  start_gas: 2,                                                                               │
 │}                                                                                             │
 │                                                                                              │
 │InterchangeEntry {                                                                            │
 │  operator: +,                                                                                │
 │  operands: [ ],                                                                              │
 │  output_scheme: (Int -> (Int -> Int)),                                                       │
 │  output_flat_value: <<closure>>,                                                             │
 │  start_gas: 2,                                                                               │
 │}                                                                                             │
 │                                                                                              │
 │InterchangeEntry {                                                                            │
 │  operator: +,                                                                                │
 │  operands: [ InterchangeOperand(uhCkkboI9VZ_V1s5pM0j97vF66wcThve2QRs_n9VlxsUk9o_SgYkg), ],   │
 │  output_scheme: (Int -> Int),                                                                │
 │  output_flat_value: <<closure>>,                                                             │
 │  start_gas: 2,                                                                               │
 │}                                                                                             │
 │                                                                                              │
 └──────────────────────────────────────────────────────────────────────────────────────────────┘

I find the pretty crate to have some weird edge cases, but overall it seems to perform its job of laying things out depending on their width (relative to total frame width) ok.