Automatically pump os.proc streams when SystemStreams are redirected

[lihaoyi]

Depends on com-lihaoyi/os-lib#283

This moves the subprocess stream handling logic out of Jvm.spawnSubprocess and makes it apply to all os.proc invocations, greatly reducing the room for error. With this, Jvm.spawnSubprocess becomes a very thin wrapper around os.proc.spawn. We also rely directly on OS-Lib's own pumper threads to pump to our destination, rather than having them pump into in-memory buffers and then spawning our own pumper threads to pump from those buffers to the destination

I spent some time looking into how to do the stdout/err handling at the process level, but couldn't find any reasonable mechanism to do so that allows us to preserve the ordering of the stdout/stderr. This is the original motivation to squishing it into one stream via ProxyOutputStream/ProxyStreamPumper and is important because otherwise you find e.g. stack traces out of order with printlns, which makes debugging very difficult. Might be possible using some socket/fifo/pipe cleverness, but not as part of this PR

Added an integration test to assert on the subtleties of stdout, stderr, and their inherited alternatives. This PR is required for the test to pass

[lihaoyi]

Making these lazily-initialized is necessary to avoid infinite recursion when initializing the pumper <-> stream <-> subprocess data structure, which is circular

[lihaoyi]

I think this should be ready for review @lefou

[lefou]

Looks good to me. Due to lack of test coverage, we should manually test this at least once in a BSP session.

[lihaoyi]

Tested the BSP integration via intellij on example/scalabuilds/9-realistic. Seems to load the project successfully with some printlns and os.proc calls added to the necessary tasks