Merge #81

81: Trap handler override improvments r=almindor a=MabezDev

* Add ability to override trap handling mechanism
  * Previously, `_start_trap` was marked as weak, which when compiled
    into a static archive, that information is ignored.
  * Now by default we `PROVIDE` the default trap handler, if another one
    has not been specified from another crate.
* Mark the fields of `Vector` public, for use outside of `riscv-rt`

Co-authored-by: Scott Mabin <scott@mabez.dev>