Add *StateDB.TxHash for usage with Warp (#204)

## Why this should be merged

Coreth and Subnet-EVM require access to the current tx hash in the Warp
precompile (for selecting the correct predicate results).

This can not be exposed as a wrapper through the `OverrideEVMResetArgs`
because `SetTxContext` is called _prior_ to resetting the EVM instance.
Therefore, the `SetTxContext` function on any DB wrapper would never be
called.

## How this works

Exposes the existing `txHash` through a `TxHash()` function.

## How this was tested

Added a trivial unit test.

core/state/statedb.libevm.go

@@ -24,6 +24,11 @@ import (
 	"github.com/ava-labs/libevm/libevm/stateconf"
 )
 
+// TxHash returns the current transaction hash set by [StateDB.SetTxContext].
+func (s *StateDB) TxHash() common.Hash {
+	return s.thash
+}
+
 // SnapshotTree mirrors the functionality of a [snapshot.Tree], allowing for
 // drop-in replacements. This is intended as a temporary feature as a workaround
 // until a standard Tree can be used.

core/state/statedb.libevm_test.go

@@ -36,6 +36,18 @@ import (
 	"github.com/ava-labs/libevm/triedb/hashdb"
 )
 
+func TestTxHash(t *testing.T) {
+	db := NewDatabase(rawdb.NewMemoryDatabase())
+	state, err := New(types.EmptyRootHash, db, nil)
+	require.NoError(t, err)
+
+	assert.Zero(t, state.TxHash(), "Tx hash should initially be uninitialized")
+
+	hash := common.Hash{1}
+	state.SetTxContext(hash, 3)
+	assert.Equal(t, hash, state.TxHash(), "Tx hash should have been updated")
+}
+
 func TestStateDBCommitPropagatesOptions(t *testing.T) {
 	memdb := rawdb.NewMemoryDatabase()
 	trieRec := &triedbRecorder{Database: hashdb.New(memdb, nil, &trie.MerkleResolver{})}