Fix bug with unsigned types and negative factors

[cpctaylo]

This PR fixes a bug in how the library chooses Rust types to represent signals.

We tried a DBC with a 16 bit unsigned signal with a factor of -1. This means the range of valid values is [-65535, 0]. But this confused the function scaled_signal_to_rust_int which chose i8 when it should have picked i32.

This PR will:

• Change scaled_signal_to_rust_int to look at the whole range of values for a signal and pick the smallest Rust type.
• Ignore min and max values when deciding Rust types for a signal because Config.check_ranges can disable range checking. To be on the safe side, this picks types assuming the worst-case values for a signal.
• Drop the symlink in testing/can-messages/src/messages.rs because it doesn't work on Windows. Now testing/can-messages/build.rs copies the file from can-messages to can-embedded.
• Fix build order so can-messages goes before can-embedded.
• Add unit and integration tests. UnsignedNegativeFactorSignal in example.dbc will reproduce the bug.

Happy to address any comments or change requests.

[cplnixon]

Implemented logic seems sound and test coverage looks thorough 👍

[killercup]

I trust you guys!