Introduce semgrep

Introduce usage of semgrep by doing:

- Add a `semgrep` config file in `contrib/semgrep.yml`.
- Add a single rule to check that all `From` impls are inlined

Author: tcharding

contrib/semgrep.yml

@@ -0,0 +1,11 @@
+rules:
+
+- id: inline-from-impls
+  patterns:
+    - pattern: |-
+        impl From<$F> for $T { fn from(...) -> $SELF { ... } }
+    - pattern-not-regex: "inline"
+  message: Use inline attribute on `From` implementations.
+  languages:
+    - rust
+  severity: ERROR