implement From<Frame> for BacktraceFrame

There are situations where we can only capture raw `Frame` (example:
capturing from a signal handler), but it could still be that we can
process them later and are not fully nostd in the environment.

With a conversion from Frame to BacktraceFrame, this is trivial. But
without it, we can't really use the pretty printers as they are reliant
on BacktraceFrame, not Frame.

Fixes: #419

Author: glommer

src/capture.rs

@@ -249,6 +249,15 @@ impl From<Vec<BacktraceFrame>> for Backtrace {
     }
 }
 
+impl From<crate::Frame> for BacktraceFrame {
+    fn from(frame: crate::Frame) -> BacktraceFrame {
+        BacktraceFrame {
+            frame: Frame::Raw(frame),
+            symbols: None,
+        }
+    }
+}
+
 impl Into<Vec<BacktraceFrame>> for Backtrace {
     fn into(self) -> Vec<BacktraceFrame> {
         self.frames