channel padding: Make Parameters a pub struct with builder

chanmgr is going to want to make one of these from a NetDir.

Cargo.lock

@@ -3869,6 +3869,7 @@ dependencies = [
  "bytes",
  "cipher",
  "coarsetime",
+ "derive_builder_fork_arti",
  "digest 0.10.3",
  "educe",
  "futures",

crates/tor-proto/Cargo.toml

@@ -25,6 +25,7 @@ asynchronous-codec = "0.6.0"
 bytes = "1"
 cipher = "0.4.1"
 coarsetime = "0.1.20"
+derive_builder = { version = "0.11.2", package = "derive_builder_fork_arti" }
 digest = "0.10.0"
 educe = "0.4.6"
 futures = "0.3.14"

crates/tor-proto/src/channel.rs

@@ -59,7 +59,7 @@ pub const CHANNEL_BUFFER_SIZE: usize = 128;
 mod circmap;
 mod codec;
 mod handshake;
-mod padding;
+pub mod padding;
 mod reactor;
 mod unique_id;
 

crates/tor-proto/src/channel/padding.rs

@@ -6,6 +6,7 @@ use std::pin::Pin;
 // TODO, coarsetime maybe?  But see arti#496 and also we want to use the mockable SleepProvider
 use std::time::{Duration, Instant};
 
+use derive_builder::Builder;
 use educe::Educe;
 use futures::future::{self, FusedFuture};
 use futures::FutureExt;
@@ -91,14 +92,24 @@ pub(crate) struct Timer<R: SleepProvider> {
 }
 
 /// Timing parameters, as described in `padding-spec.txt`
-#[derive(Debug, Copy, Clone, Eq, PartialEq)]
-pub(crate) struct Parameters {
+#[derive(Debug, Copy, Clone, Eq, PartialEq, Builder)]
+pub struct Parameters {
     /// Low end of the distribution of `X`
+    #[builder(default = "1500")]
     pub(crate) low_ms: u32,
     /// High end of the distribution of `X` (inclusive)
+    #[builder(default = "9500")]
     pub(crate) high_ms: u32,
 }
 
+impl Default for Parameters {
+    fn default() -> Self {
+        ParametersBuilder::default()
+            .build()
+            .expect("could not build default channel padding Parameters")
+    }
+}
+
 /// Timing parameters, "compiled" into a form which can be sampled more efficiently
 ///
 /// According to the docs for [`rand::Rng::gen_range`],