Adds the `result` module to contain types for fallible functions.

For discussion, see:
<rust-embedded#212 (comment)>

Adds fallible `try_` function variants to most `register` module macros.

Adds fallible access functions for `Mcounteren` HPM fields.

Adds fallible access functions for `Scounteren` HPM fields.

Adds fallible access functions for `Mcountinhibit` HPM fields.

Adds fallible access functions for `Satp` ASID and PPN fields.

Adds fallible conversion functions for `Pmpcfgx` permission and range
fields.

Uses the new `Result` type for library traits.

Uses the `riscv_pac::result::*` types for unsafe trait implementations.

Changes current `Invalid*` variants to `InvalidField*` variants to
indicate their use for subfields in an outer struct.

Adds the field-less counterparts for use in a context without subfields.