Huge speedup of RemoteMirror testing

**Background:**

Each RemoteMirror test runs two processes: A host process runs the
RemoteMirror library and queries memory data from a target process.  The host
and target communicate by passing strings back and forth with requests to read
particular information in memory.  The target is pure Swift; the host is
C/C++.

**What this change does:**

Without this, the host makes very many small requests to the target.
Each of those requests has to be individually parsed by the target
using String operations from the debug stdlib.  Actually transferring
the raw bytes is relatively quick.
With this, the host requests and caches "pages" of about 4k and
satisfies most requests from previously-fetched data.  This dramatically
reduces the total number of operations.

**Performance notes:**

The following notes only count the time to compile and run the 78 tests in the
validation-tests/Reflection directory using `lit.py`; it does not include time
to rebuild the project before running tests.

**Performance with debug stdlib:**

(That is: `build-script -ra --debug-swift-stdlib`) Before this change, I got
tired of waiting after about 15 minutes and it was about 1/4 done.  Some very
simplistic profiling showed >99% of the time being spent in stdlib String
operations in the target process.  Activity Monitor shows that individual tests
run for about 6 minutes of CPU time each.

With this change, the tests run in about 70s of wall time.  Almost all of the
time seems to spent compiling the tests; the tests themselves run too quickly to
even show up in the Activity Monitor.

**Performance with release stdlib:**

(That is: `build-script -ra`)
The tests run in about 70s of wall time with or without this change.

Author: tbkka