Add Support for 74HC595 Serial -> Parallel Shift Register

[gsexton]

The 74HC595 converts a serial data stream to parallel output on it's data pins. The Adafruit I2C/SPI backpack uses this device for the SPI side of the backpack.

I'll be adding an SPI backpack to the HD44780 LCD display that uses this.

[codecov]

Codecov Report

Attention: Patch coverage is 49.05660% with 54 lines in your changes missing coverage. Please review.

Project coverage is 51.8%. Comparing base (f007d15) to head (b509627).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
nxp74hc595/74hc595.go	51.7%	42 Missing ⚠️
nxp74hc595/pin.go	36.8%	12 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main     #99     +/-   ##
=======================================
- Coverage   51.8%   51.8%   -0.0%     
=======================================
  Files        102     104      +2     
  Lines      13073   13179    +106     
=======================================
+ Hits        6772    6826     +54     
- Misses      6009    6061     +52     
  Partials     292     292             

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[maruel]

Thanks!