Provide better errors when deserializing build.toml (#145)

Before this change when there was an error in `build.toml`, one would
get a generic error:

```
Error: Cannot parse TOML in /Users/daniel/git/relu-metal/build.toml

Caused by:
    data did not match any variant of untagged enum BuildCompat
```

However, what we want is to give the error when trying to parse as the
V2 format, since the V1 format is only there for backwards
compat/updates. With this change the error of parsing as V2 is bubbled
up. For instance:

```
Error: Cannot parse TOML in /Users/daniel/git/relu-metal/build.toml

Caused by:
    Unknown variant metal. Expected one of cuda, rocm
```

Author: danieldk

build2cmake/Cargo.lock

@@ -19,6 +19,7 @@ minijinja-embed = "2.5"
 rand = "0.8"
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
+serde-value = "0.7"
 toml = "0.8"
 
 [build-dependencies]

build2cmake/Cargo.toml

@@ -4,15 +4,33 @@ use serde::Deserialize;
 pub mod v1;
 
 mod v2;
+use serde_value::Value;
 pub use v2::{Backend, Build, Dependencies, Kernel, Torch};
 
-#[derive(Debug, Deserialize)]
-#[serde(untagged)]
+#[derive(Debug)]
 pub enum BuildCompat {
     V1(v1::Build),
     V2(Build),
 }
 
+impl<'de> Deserialize<'de> for BuildCompat {
+    fn deserialize<D>(deserializer: D) -> std::result::Result<Self, D::Error>
+    where
+        D: serde::Deserializer<'de>,
+    {
+        let value = Value::deserialize(deserializer)?;
+
+        match v1::Build::deserialize(value.clone()) {
+            Ok(v1_build) => Ok(BuildCompat::V1(v1_build)),
+            Err(_) => {
+                let v2_build: Build =
+                    Build::deserialize(value).map_err(serde::de::Error::custom)?;
+                Ok(BuildCompat::V2(v2_build))
+            }
+        }
+    }
+}
+
 impl TryFrom<BuildCompat> for Build {
     type Error = eyre::Error;