Add `transaction::fees::zip317::MINIMUM_FEE` constant and

`transaction::components::amount::Amount::const_from_i64`.

Signed-off-by: Daira Emma Hopwood <daira@jacaranda.org>

zcash_primitives/CHANGELOG.md

@@ -67,6 +67,10 @@ and this library adheres to Rust's notion of
 - `merkle_tree::read_incremental_witness` replaces `merkle_tree::IncrementalWitness::read`
 - `merkle_tree::merkle_path_from_slice` replaces `merkle_tree::MerklePath::from_slice`
 - `sapling::{CommitmentTree, IncrementalWitness, MerklePath, NOTE_COMMITMENT_TREE_DEPTH}`
+- `transaction::fees::zip317::MINIMUM_FEE`, reflecting the minimum possible
+  [ZIP 317](https://zips.z.cash/zip-0317) conventional fee.
+- `transaction::components::amount::Amount::const_from_i64`, intended for constructing
+  a constant `Amount`.
 
 ### Changed
 - The bounds on the `H` parameter to the following methods have changed:

zcash_primitives/src/transaction/components/amount.rs

@@ -32,6 +32,14 @@ impl Amount {
         Amount(0)
     }
 
+    /// Creates a constant Amount from an i64.
+    ///
+    /// Panics: if the amount is outside the range `{-MAX_MONEY..MAX_MONEY}`.
+    pub const fn const_from_i64(amount: i64) -> Self {
+        assert!(-MAX_MONEY <= amount && amount <= MAX_MONEY); // contains is not const
+        Amount(amount)
+    }
+
     /// Creates an Amount from an i64.
     ///
     /// Returns an error if the amount is outside the range `{-MAX_MONEY..MAX_MONEY}`.

zcash_primitives/src/transaction/fees/zip317.rs

@@ -13,6 +13,13 @@ use crate::{
     },
 };
 
+/// The minimum conventional fee using the standard [ZIP 317] constants. Equivalent to
+/// `(FeeRule::standard().marginal_fee() * FeeRule::standard().grace_actions()).unwrap()`,
+/// but as a constant.
+///
+/// [ZIP 317]: https//zips.z.cash/zip-0317
+pub const MINIMUM_FEE: Amount = Amount::const_from_i64(10_000);
+
 /// A [`FeeRule`] implementation that implements the [ZIP 317] fee rule.
 ///
 /// This fee rule supports only P2pkh transparent inputs; an error will be returned if a coin