Picollo: Modern HDR histogram and PMU counters for .NET

Picollo is a new library for serious performance work in .NET. Picollo stands for Performance Instrumentation and Continuous Observation for Low-Level Optimization. The initial public release of Picollo v0.1.0 contains two components. First, a modern version of an HDR histogram, which is much faster for recording data and easier to use. Second, a set of APIs for `perf_event_open`, including fast-path reads, that give raw access to PMU counters from .NET on Linux, WSL included.