Add privacy boundary to BlockTime

As per policy in #4090 add a privacy boundary to the `BlockTime` type.
Use the module name `encapsulate` as is done in `amount` - its private
so the name can easily be changed later if needed.

Explicitly do not run the formatter or update rustdoc column width so
that review is easier.

units/src/time.rs

@@ -9,6 +9,8 @@
 
 #[cfg(feature = "arbitrary")]
 use arbitrary::{Arbitrary, Unstructured};
+
+mod encapsulate {
     #[cfg(feature = "serde")]
     use serde::{Deserialize, Serialize};
 
@@ -37,6 +39,9 @@ impl BlockTime {
         #[inline]
         pub const fn to_u32(self) -> u32 { self.0 }
     }
+}
+#[doc(inline)]
+pub use encapsulate::BlockTime;
 
 impl From<u32> for BlockTime {
     #[inline]