Integrate support for sanitizers.

Use polysquare_bootstrap_sanitizers to enable sanitizers in a project.
Sanitized binaries are disabled by default, but you can use
POLYSQUARE_USE_(ASAN|MSAN|TSAN|UBSAN) to enable them.

When sanitizers are enabled, targets will be mirrored such that each
target will have a correspending target_(asan|msan|tsan|ubsan) and be
linked to other corresponding sanitized targets.

Projects can use NO_(ASAN|MSAN|TSAN|UBSAN) to disable that form of sanitization
on a target.