Flash Read/Write Support (#257)

* Added Flash Read/Write Support

Incorporated Comments

 - Added FlashReaderGuard
 - Removed modulo operations
 - Added option to avoid verify operations
 - Cleanup

Added more detailed notes for usage of unsafe in flash functions.

ran "cargo fmt" to cleanup flash code

Suppressed unused warnings for flash code, enum naming tweaks

Added prefix '_' to registers and constants that aren't currently used,
and renamed enum values based on review feedback

added more descriptive comments where requested

moved initialization of hword closer to where it's used

Removed FlashReaderGuard, not necessary

We can rely on the lifetime system to protect us here, the guard
structure is unnecessary (see the unsafe comments for details).

removed lifetimes from FlashWriter::read()

Better to let the compiler determine the lifetime

Co-authored-by: Cor <prive@corpeters.nl>

Author: irwineffect

CHANGELOG.md

@@ -25,6 +25,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 ### Added
 
 - Add runtime-reconfigurable GPIO pins
+- Added support for writing/reading/erasing onboard flash
 
 ### Fixed