Entry API for cuprate_database

[hinto-janai]

What

This proposal describes a new std::collections::btree_map::Entry-like API to be added to cuprate_database.

3 public types will be added:

• enum Entry
• struct OccupiedEntry
• struct VacantEntry

The API for these types will closely resemble std's API.

Where

Added onto the cuprate_database::DatabaseRw trait:

// where
//     T: Table
fn entry(&mut self, key: &T::Key) -> Entry<'_, T>;

This replaces functions like DatabaseRw::update:

cuprate/storage/cuprate-blockchain/src/database.rs

Line 195 in b76042a

fn update<F>(&mut self, key: &T::Key, mut f: F) -> Result<(), RuntimeError>

and some awkward code:

cuprate/storage/cuprate-blockchain/src/ops/output.rs

Lines 32 to 41 in b76042a

	// FIXME: this would be much better expressed with a
	// `btree_map::Entry`-like API, fix `trait DatabaseRw`.
	let num_outputs = match tables.num_outputs().get(&amount) {
	// Entry with `amount` already exists.
	Ok(num_outputs) => num_outputs,
	// Entry with `amount` didn't exist, this is
	// the 1st output with this amount.
	Err(RuntimeError::KeyNotFound) => 0,
	Err(e) => return Err(e),
	};

Why

All the same reasons why std::collections::btree_map::Entry exists.