rust: Move `incremental_sinsemilla_tree_ffi` into crate root

Having this be a submodule of `orchard_ffi` while following Rust 2018
module structure made it impossible to use the `include!` macro on
`orchard_ffi.rs`, which is exactly what the `zcash_script` crate does.
See this comment for details:
    https://github.com/rust-lang/rust/issues/50132#issuecomment-969450868

To resolve this, we restructure the FFI library crate to only have FFI
methods in "leaf" module files.

src/rust/src/incremental_sinsemilla_tree_ffi.rs

@@ -6,13 +6,13 @@ use std::mem::size_of_val;
 use std::ptr;
 
 use orchard::{bundle::Authorized, tree::MerkleHashOrchard};
-
+use tracing::error;
 use zcash_primitives::{
     merkle_tree::incremental::{read_frontier_v1, read_tree, write_frontier_v1, write_tree},
     transaction::components::Amount,
 };
 
-use crate::orchard_ffi::{error, CppStreamReader, CppStreamWriter, ReadCb, StreamObj, WriteCb};
+use crate::streams_ffi::{CppStreamReader, CppStreamWriter, ReadCb, StreamObj, WriteCb};
 
 pub const MERKLE_DEPTH: u8 = 32;
 pub const MAX_CHECKPOINTS: usize = 100;

src/rust/src/orchard_ffi.rs

@@ -19,8 +19,6 @@ use zcash_primitives::transaction::{
 
 use crate::streams_ffi::{CppStreamReader, CppStreamWriter, ReadCb, StreamObj, WriteCb};
 
-mod incremental_sinsemilla_tree_ffi;
-
 #[no_mangle]
 pub extern "C" fn orchard_bundle_clone(
     bundle: *const Bundle<Authorized, Amount>,

src/rust/src/rustzcash.rs

@@ -70,6 +70,7 @@ mod tracing_ffi;
 
 mod address_ffi;
 mod history_ffi;
+mod incremental_sinsemilla_tree_ffi;
 mod orchard_ffi;
 mod transaction_ffi;
 mod zip339_ffi;