remove trait bounds from sans-IO methods, for flexibility in extension crates

[danjl1100]

Motivation

Libraries providing extensions traits for different devices using the ht16k33 (e.g. adafruit-7segment) become infected with HAL-specific trait bounds.

Solution

This PR moves the sans-IO methods to a separate impl block with no restriction on the I2C parameter (e.g. not tied to a specific HAL trait).

The thought being: if someone wants to use a different HAL, then they can patch only this crate ht16k33 and not every crate that extends its' functionality through data manipulation helpers. (e.g. patch ht16k33, and not both ht16k33 and adafruit-7segment)

The change is small (please check the diff of the "removed"/"added" sections), but let me know if you have any issues with merging this in.

NOTE: I left the new and destroy functions inside the HAL trait bounds (conservative approach) since the end user will still need to satisfying the trait bounds when constructing the display.