re-implement writing to serial port

This functionality was removed when Pipe (sync-producer-async-consumer) was introduced. It is now re-implemented.

[fix] use the new sync-producer-async-consumer impl in main

v0.4.1

replace the ad hoc sync-producer-async-consumer in main with a Pipe

new implementation of sync-producer-async-consumer

This fixes the handling of promises (e.g. requests) that resolve out-of-order. The sync-producer-async-consumer has been defined as the Pipe abstraction, and an implementation has been written using an async loop that uses Lwt.pick across three concurrent tasks.
Tests have been added that verify two scenarios are handled as expected: promises resolving out-of-order are handled in order of resolution, and async handling of promises does not block on an empty channel between the producer and consumer.

Connection: close header adder to request loads

v0.3.9

add Connection: close http header to all requests in a Load.t

fix Cohttp leaking unix file descriptors

v0.3.8

fix unix fd leak caused by Cohttp

handle log dir already exists

v0.3.7

handle dir already exists

timestamped log file names to specified log dir

v0.3.6

write to log-dir with timestamp filename

optionally write to log file

v0.3.5

optionally write to log file

write single byte per response (unless in -serial-debug mode)

v0.3.4

read one byte from serial and add error handling

rectwave, uncomment error handling, serial-debug flag

v0.3.3

add flag to optionally send debug info over serial