Improve Cover HTML page

OTP-15213

* Adapt diff colors to bring more contrast
* Use monospace font for all the code in cover HTML report
* Specify background color for the body as white
* Reduce font-size for header
* Install cover stylesheet when making release (@sirihansen made this improvement)
* Additional changes based on feedback

This makes the nightly tests slightly more stable as they assert
that the server isn't alive when lcnt:start/0 is called, which it
could still be since the stop command was a plain gen_server call
that didn't wait until the termination was completed.

This commit replaces the old memory instrumentation with a new
implementation that scans carriers instead of wrapping
erts_alloc/erts_free. The old implementation could not extract
information without halting the emulator, had considerable runtime
overhead, and the memory maps it produced were noisy and lacked
critical information.

Since the new implementation walks through existing data structures
there's no longer a need to start the emulator with special flags to
get information about carrier utilization/fragmentation. Memory
fragmentation is also easier to diagnose as it's presented on a
per-carrier basis which eliminates the need to account for "holes"
between mmap segments.

To help track allocations, each allocation can now be tagged with
what it is and who allocated it at the cost of one extra word per
allocation. This is controlled on a per-allocator basis with the
+M<S>atags option, and is enabled by default for binary_alloc and
driver_alloc (which is also used by NIFs).