bit_machine: return concrete type from as_bit_iter

Returning a concrete type lets us get more trait methods rather than just
`Iterator`. In particular we should be able to get `ExactSizeIterator`
and `Fuse` which are important for efficiency.

We also want to name this type elsewhere, and even the existential one
was annoying to type, so we add an alias for it.

Author: apoelstra

src/bit_machine/frame.rs

@@ -104,7 +104,7 @@ impl Frame {
 
     /// Extend the present frame with a read-only reference the the data
     /// and return the resulting struct.
-    pub fn as_bit_iter<'a>(&self, data: &'a [u8]) -> BitIter<impl Iterator<Item = u8> + 'a> {
+    pub(super) fn as_bit_iter<'a>(&self, data: &'a [u8]) -> super::FrameIter<'a> {
         BitIter::byte_slice_window(data, self.start, self.start + self.len)
     }
 }

src/bit_machine/mod.rs

@@ -24,6 +24,9 @@ use simplicity_sys::ffi::UWORD;
 
 pub use self::limits::LimitError;
 
+/// An iterator over the contents of a read or write frame which yields bits.
+pub type FrameIter<'a> = crate::BitIter<core::iter::Copied<core::slice::Iter<'a, u8>>>;
+
 /// An execution context for a Simplicity program
 pub struct BitMachine {
     /// Space for bytes that read and write frames point to.