Miscellaneous cleanup and refactoring

[jessebraham]

Sorry, this started off with a couple little changes and grew into a bit of a mess 😅 Reviewing commit-by-commit will probably be easier.

There's really nothing interesting in this PR, but the changes essentially boil down to:

• Simplify/improve the build script, less duplication
• Refactor and clean up the io module
• Alias Target type with a properly capitalized name (I thought it was a module initially)
• Move the Transport type definitions into the library
• Change the bit_buf struct field from u64 to u32 (closes #38)
• Assorted cleanup/updates/improvements

Tested with espflash:

Chip	UART	USB Serial JTAG
ESP32	✅	N/A
ESP32-C2	✅	N/A
ESP32-C3	✅	✅
ESP32-C6	✅	✅
ESP32-H2	✅	✅
ESP32-S2	✅	N/A
ESP32-S3	✅	✅