Refactor: cleanup before updating FinalizeSettle

[fedgiac]

Small changes that I did while writing PR #47 that can stand alone.
I separated them to make that PR smaller. (It's still quite large unfortunately.)
Nothing in this PR changes the behavior of any code or test.

Changes

• Shared order-building helpers to new file. This will be used in the FinalizeSettle as well.
• Rename the client's begin-side input struct: SettledOrder to InitializedIntent. This is because we're going to introduce FinalizedIntent, which serves a similar role and would be more fitting for the name SettledOrder anyway but loses the symmetry with InitializedIntent.
• ix_data! is used more in some of the BeginSettle encoding tests.
• Drop a redundant re-export

interface/src/instruction/settle/mod.rs re-exported SettledOrders, but nothing outside the module refers to it — it's only ever used internally as the type of BeginSettleInput::orders. Removed it from the pub use line.

How to test

CI.
You can also do git diff --find-renames --color-moved HEAD~1 to see from the color coding that the new file is mostly copied lines apart from comments and pub.

[kaze-cow]

changes are easy to follow! I like the new names for interface/client objects.