Switch to Rust 1.49.0

Notable improvements:
- [track_caller](https://github.com/rust-lang/rfcs/blob/master/text/2091-inline-semantic.md): We can get the location of the caller of a message, which makes logging easier as you can have dedicated functions (not only macros), e.g. write a function `log(self: Result, msg: string) -> Option`
- The output after a panic is shorter and hides lots of uninteresting
function calls
- Test output is better captured

After merging, we should probably wait a bit with using the track_caller
feature to make going back easier, should we get problems again.

rust-toolchain

@@ -1 +1 @@
-1.45.0
+1.49.0