Adding on top of the new builder pattern for creating XCM programs, I'm
adding some more APIs:
```rust
let paying_fees: Xcm<()> = Xcm::builder() // Only allow paying for fees
.withdraw_asset() // First instruction has to load the holding register
.buy_execution() // Second instruction has to be `buy_execution`
.build();
let paying_fees_invalid: Xcm<()> = Xcm::builder()
.withdraw_asset()
.build(); // Invalid, need to pay for fees
let not_paying_fees: Xcm<()> = Xcm::builder_unpaid()
.unpaid_execution() // Needed
.withdraw_asset()
.deposit_asset()
.build();
let all_goes: Xcm<()> = Xcm::builder_unsafe() // You can do anything
.withdraw_asset()
.deposit_asset()
.build();
```
The invalid bits are because the methods don't even exist on the types
that you'd want to call them on.
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Co-authored-by: command-bot <>
Francisco Aguirre