Split {format} and allow use of {binary-ext} in pkg-url (#95)

This from feedback in #19:

> wrt. bin-dir and bin-path, this appears to be a typo / should all be called bin-dir

This is only a readme fix afaict, I changed all occurences of `bin-path` in there to `bin-dir`.

> wrt. format, those are actually two (unfortunately named) different concepts, the first
refers to the archive format (eg. .tgz), the second to the binary format (which needs a .exe
appended for windows).

This introduces two new substitutions:
- `binary-ext` is the old "`format` in `bin-dir`"
- `archive-format` is the old "`format` in `pkg-url`"

Contents are unchanged: `binary-ext` includes the dot, `archive-format` doesn't. That
makes it easy to upgrade and also personally I slightly prefer it that way.

The old contextual `format` is still available, "soft deprecated": it will be accepted silently
so everything will work, but all documentation will use the new syntax. In the future we
could move to a "hard deprecated" model where installing a package that uses `format`
will warn the user / tell them to report that to the maintainer. I don't think we'll ever really
be able to remove it but that should be good enough.

A cool new feature is that `binary-ext` is now usable in `pkg-url`, which will be useful for raw binary downloads:

```toml
pkg_url = "{ repo }/releases/download/v{ version }/{ name }-v{ version }-{ target }{ binary-ext }"
```

I've also added a bunch of tests to GhCrateMeta around the templating for `pkg-url`.

src/bins.rs

@@ -16,18 +16,20 @@ impl BinFile {
     pub fn from_product(data: &Data, product: &Product) -> Result<Self, anyhow::Error> {
         let base_name = product.name.clone().unwrap();
 
-        // Generate binary path via interpolation
+        let binary_ext = if data.target.contains("windows") {
+            ".exe"
+        } else {
+            ""
+        };
+
         let ctx = Context {
             name: &data.name,
             repo: data.repo.as_ref().map(|s| &s[..]),
             target: &data.target,
             version: &data.version,
-            format: if data.target.contains("windows") {
-                ".exe"
-            } else {
-                ""
-            },
             bin: &base_name,
+            format: binary_ext,
+            binary_ext,
         };
 
         // Generate install paths
@@ -39,8 +41,8 @@ impl BinFile {
             data.bin_path.join(&source_file_path)
         };
 
-        // Destination path is the install dir + base-name-version{.format}
-        let dest_file_path = ctx.render("{ bin }-v{ version }{ format }")?;
+        // Destination path is the install dir + base-name-version{.extension}
+        let dest_file_path = ctx.render("{ bin }-v{ version }{ binary-ext }")?;
         let dest = data.install_path.join(dest_file_path);
 
         // Link at install dir + base name
@@ -118,8 +120,14 @@ struct Context<'c> {
     pub repo: Option<&'c str>,
     pub target: &'c str,
     pub version: &'c str,
-    pub format: &'c str,
     pub bin: &'c str,
+
+    /// Soft-deprecated alias for binary-ext
+    pub format: &'c str,
+
+    /// Filename extension on the binary, i.e. .exe on Windows, nothing otherwise
+    #[serde(rename = "binary-ext")]
+    pub binary_ext: &'c str,
 }
 
 impl<'c> Template for Context<'c> {}