error on any invalid target combination

[hazelutf8]

Only allow exactly one target "feature" to be set at any time, as discussed in #112 and discord.
Otherwise a compile-time error message will be given.

Is the "greater than one target specified" case useful to error on as well?
The "zero target specified" case has a confusing message about missing imports and methods, but the compiler has a message which might already be close enough.

error[E0252]: the name `pac` is defined multiple times
   --> src\lib.rs:104:9
    |
101 | pub use stm32f1::stm32f100 as pac;
    |         ------------------------- previous import of the module `pac` here
...
104 | pub use stm32f1::stm32f101 as pac;
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^ `pac` reimported here
    |
    = note: `pac` must be defined only once in the type namespace of this module
help: you can use `as` to change the binding name of the import
    |
104 | pub use stm32f1::stm32f101 as other_pac;
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error

Is only the first "zero targets specified" error message wanted?
Feedback is welcome.

[therealprof]

LGTM as it, thanks!