add netstring+Stream utility

[warner]

What is the Problem Being Solved?

The kernel-to-subprocess protocol uses netstrings to frame each message. In #1797 I discovered that the library we were using for this fails to handle partial chunks in the stream decoder, which breaks the protocol any time the delivery message is larger than the OS kernel's pipe buffer (typically about 8kb, but it varies by OS, which is why the test passed on Linux but failed on macOS).

I haven't been able to find any correct libraries that offer a Stream interface, so I have to write my own.

Description of the Design

Four functions:

• encode: takes a Buffer, returns a Buffer
• decode: takes a Buffer, returns a list of Buffers (for the completed netstrings) plus a leftover Buffer
• createWriter: creates a Transform stream that takes Buffers (each of which is a single payload) and emits Buffers (to be concatenated out to a byte pipe)
• createReader: creates a Transform stream that takes Buffers (which are chunks that do not necessarily align to netstring boundaries, but when all are received and concatenated, will form one or more netstrings) and emits Buffers (one payload each)

Security Considerations

The usual memory-consumption attack exists (the sender just writes 99999.. forever, and the recipient keeps buffering the supposed length prefix forever in hopes of seeing the termination colon). We control both sides of the pipe, however, so I'm not going to try to address it.

Test Plan

Unit tests.