Support an annotation for generating bitfield sizes in C code (: SIZE)

Closes #609
Closes #611

tests/rust/bitfield/Cargo.lock

@@ -0,0 +1,5 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+[[package]]
+name = "bitfield"
+version = "0.1.0"

tests/rust/bitfield/Cargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "bitfield"
+version = "0.1.0"
+authors = ["cbindgen"]
+
+[features]
+cbindgen = []

tests/rust/bitfield/cbindgen.toml

@@ -0,0 +1,3 @@
+[parse.expand]
+crates = ["bitfield"]
+features = ["cbindgen"]

tests/rust/bitfield/src/lib.rs

@@ -0,0 +1,15 @@
+#[repr(C)]
+pub struct HasBitfields {
+    #[cfg(not(feature = "cbindgen"))]
+    foo_and_bar: u64,
+
+    #[cfg(feature = "cbindgen")]
+    /// cbindgen:bitfield=8
+    foo: u64,
+    #[cfg(feature = "cbindgen")]
+    /// cbindgen:bitfield=56
+    bar: u64,
+}
+
+#[no_mangle]
+pub extern "C" fn root(_: &HasBitfields) {}