hardware protocol v2: frame layer and shared symbol registry

Adds the transport-independent core of the MaSzyna Hardware Protocol v2:
COBS framing with a CRC32C protected 24 byte header, bounds checked
payload readers and writers, and a resynchronising stream decoder.

The shared TrainState registry describes simulator state in engineering
units with per-value quality, the command registry generates the command
namespace from Commands_descriptions, and the symbol registry merges both
into the public, name-addressed hardware api.

hardware/registry/hardware_symbol_registry.h

@@ -0,0 +1,64 @@
+/*
+This Source Code Form is subject to the
+terms of the Mozilla Public License, v.
+2.0. If a copy of the MPL was not
+distributed with this file, You can
+obtain one at
+http://mozilla.org/MPL/2.0/.
+*/
+
+#pragma once
+
+#include <limits>
+#include <optional>
+#include <string>
+#include <unordered_map>
+#include <vector>
+
+#include "hardware/protocol/protocol_defs.h"
+
+// public symbol table of the hardware api
+// merges the state and command registries into one namespace-prefixed list; devices refer to symbols
+// by these names and receive short, session scoped handles in return
+
+namespace hardware
+{
+
+struct symbol_descriptor
+{
+	// full public name, i.e. "state.velocity" or "command.trainbrakeset"
+	std::string name;
+	symbol_kind kind = symbol_kind::state;
+	value_type type = value_type::float32;
+	std::uint8_t access = access_read;
+	std::string unit;
+	std::optional<double> minimum;
+	std::optional<double> maximum;
+	// index into the registry the symbol originates from
+	std::size_t source_index = 0;
+};
+
+class symbol_registry
+{
+
+public:
+// types
+	static constexpr std::size_t npos = std::numeric_limits<std::size_t>::max();
+
+// methods
+	static symbol_registry const &instance();
+
+	std::vector<symbol_descriptor> const &entries() const { return m_entries; }
+	// returns index of the symbol, or npos if the name isn't known
+	std::size_t find( std::string const &Name ) const;
+
+private:
+// methods
+	symbol_registry();
+
+// members
+	std::vector<symbol_descriptor> m_entries;
+	std::unordered_map<std::string, std::size_t> m_index;
+};
+
+} // namespace hardware