Support for HID-based partial super-fast flashing (#523)

* fix bug

* Fixed an issue where the Game of Life menu item was not appearing (#497)

* Starting on dapjs flashing

* Adding dapjs

* Connected

* Flashing works

* Double buffer flashing

* Add SHA computation function

* Run SHA code

* Swap SHA for murmur+crc

* Switch to dual murmur3

* Partial flashing works

* Remove unused code

* Move flashing code to external/sha

* Fix whitespace

* Cleanup binary genration scripts

* Add docs for hid flashing

* bump pxt-core to 0.12.132,

docs/SUMMARY.md

@@ -306,3 +306,4 @@
      * [Servo](/device/servo)
      * [Simulator](/device/simulator)
      * [Usb](/device/usb)
+     * [Flashing via HID (CMSIS-DAP)](/hidflash)

docs/docs.md

@@ -29,4 +29,5 @@
 
 * [Command Line Interface](/cli)
 * Learn about [packages](/packages)
+* [Flashing via HID (CMSIS-DAP)](/hidflash)
 

docs/hidflash.md

@@ -0,0 +1,63 @@
+# Flashing via HID (CMSIS-DAP)
+
+When the web app has access to a HID connection to the board, it can flash
+the board via the hardware debugger interface.
+The PXT localhost server can proxy HID connections (over a WebSocket),
+and native apps can access HID via various custom APIs (which are
+likely to have lower latency than the HID proxy).
+
+This is generally done via
+writing a little flashing program to the RAM, then writing the page to be
+flashed to the RAM, and then running the program. For next page, one keeps
+the flashing program, but replaces the data. Internally, the DAPLink
+software on the @boardname@ does the same.
+
+The flashing via DAP over HID is quite a bit slower than the regular
+drag&drop kind. This is because of overheads of the DAP protocol
+and the limited throughput of HID (1 packet, of maximum 64 bytes, per millisecond).
+Additionally, the DAP protocol requires every HID packet to be acknowledged
+effectively halving the bandwidth.
+Thus, typical flashing speeds (using HID proxy) are around 14k/s, with a typical
+full flash taking 15s. Theoretical maximum is around 25k/s.
+
+A custom flashing protocol, like [HF2](https://github.com/Microsoft/uf2/blob/master/hf2.md),
+can achieve around 60k/s, however this would require updates of DAPLink software,
+and is still not very fast.
+
+## Partial flashing
+
+Instead, we take care to only flash the pages that have changed.
+In typical software development only a very small fragment of the program 
+changes on every re-deployment. Additionally, most of the program is pretty
+much constant (two bootloaders, the softdevice, and the compiled C++ runtime).
+
+This is achieved by first deploying a small program which computes
+checksums of every page. Then, these checksums are read from the device
+and compared with checksums of pages of the `.hex` file to be deployed.
+Only pages which checksums that do not match are flashed.
+
+The particular checksum algorithm used is [Murmur3](https://en.wikipedia.org/wiki/MurmurHash#MurmurHash3).
+The algorithm is simplified by removing checks for unaligned data, or hashing
+the data length, since all blocks hashed are of the same, aligned length.
+
+The Murmur3 hash was chosen since it's very fast (around 4x faster than CRC32 and around 
+15x faster than SHA256). Hashing the entire flash takes about 200ms.
+
+In fact, two 32 bit Murmur3 hashes (using different starting seeds) are computed in 
+parallel, to produce a 64 bit checksum.
+
+## Hash length analysis
+
+Let's compute the probability of some people running into trouble because
+of hash collisions on pages. Assume:
+* uniform distribution of hashes
+* 10M users
+* each user programming for 50h, and flashing every 2 minutes, i.e., 1500 flashes
+* each flashing changing 10 pages
+
+With 64 bit hashes, the probability that a collision occurs
+`1 - ((2^64 - 1) / 2^64) ^ (1e7 * 1500 * 10)` which is `0.000016`
+(Bing says so; Google wrongly said `0`).
+With 32 bit, even with only 1M users we get `97%` probability of some collisions.
+The uniformity of hashes is questionable, but the `0.000016`
+gives us some wiggle room.