Add compile test for odd capacity BufEncoder

Add two compile test crates that are identical except for the capacity
of `BufEncoder`.  Odd capacity in `fail/` that must not compile and even
capacity in `pass/` that must compile.

tests/compiletests/fail/odd-capacity/Cargo.toml

@@ -0,0 +1,10 @@
+[package]
+name = "odd-capacity"
+version = "0.1.0"
+edition = "2021"
+rust-version = "1.63.0"
+
+[workspace]
+
+[dependencies]
+hex = { package = "hex-conservative", path = "../../../../" }

tests/compiletests/fail/odd-capacity/src/main.rs

@@ -0,0 +1,6 @@
+use hex::buf_encoder::BufEncoder;
+use hex::Case;
+
+// This should fail to compile because the capacity size is odd.
+// The only difference to `even-capacity` is the capacity size.
+fn main() { let _encoder = BufEncoder::<3>::new(Case::Lower); }

tests/compiletests/pass/even-capacity/Cargo.toml

@@ -0,0 +1,10 @@
+[package]
+name = "even-capacity"
+version = "0.1.0"
+edition = "2021"
+rust-version = "1.63.0"
+
+[workspace]
+
+[dependencies]
+hex = { package = "hex-conservative", path = "../../../../" }

tests/compiletests/pass/even-capacity/src/main.rs

@@ -0,0 +1,5 @@
+use hex::buf_encoder::BufEncoder;
+use hex::Case;
+
+// This should compile, ensuring that the compile error in odd-capacity is from the odd capacity.
+fn main() { let _encoder = BufEncoder::<4>::new(Case::Lower); }