4-bit SD driver

NOTE: Version 2.x has a small change to driver loading. See examples and the readme.md

Driver versions 1.x use a C++ like object instantiating to hold device state. This requires more memory than C based pointer passing.
Driver versions 2.x use regular C allocations and pointer passing to hold device state. This also has advantage of being more portable.

Can be used with Spin2, Basic, C and any languages supported by the Flexspin compiler. Propeller 2 only.

Requires Flexspin compiler version 7.0.0 or later. The example tester uses the newly added _vfs_open_fat_handle() API, replacing the older preset list of vfs device open functions. The driver itself uses newly added features around "inline" assembly.
Recommend Flexspin v7.7.2 or later. Various little fixes and additions have been made to support the driver as compiler limitations came to light.

Requires a 4-bit wired SD slot. Main restriction is the four DAT pins have to be in-order on 4-bit pin boundary. Pull-ups are also recommended. Example can be found here - https://forums.parallax.com/discussion/174988/new-sd-mode-p2-accessory-board/p1
I made a basic 4-bit SD slot, without power switch nor LED, by hand wiring a pillaged SD slot - https://forums.parallax.com/discussion/comment/1563018/#Comment_1563018
As of v1.14, schematic and photo of hand-wired SD slot are included in the zip file.

Also included is two other drivers, using 1-bit SPI mode. One is just a copy of Flexspin's built-in sdmm.cc driver repackaged to be a plugin. The other is rehash of the same that plainly bit-bashes the SPI instead of using any smartpins. Both use v1.x interface.

There is two example programs that demonstrate mounting and use of the drivers as plugins. You'll need to dig around them to eke out the details. They are defaulted to using 1-bit SPI mode driver and the boot slot pin-out.
As of v1.14, there is now four example programs. And they default to 4-bit SD mode now.

Performance wise, with lazy-CMD12 enabled, sysclock set at 300 MHz and clock divider at 3 (100 MHz SD clock), reads can exceed 40 MB/s and writes aren't far behind at maybe 38 MB/s assuming the SD card is good for it. Default divider is sysclock/4.

PS: v2.3 zip file contains both v2.3 and v1.28 drivers