add wip span type and new file2 interface and fstream impl.

testing was very minimal

solanaceae/file/file2.hpp

@@ -0,0 +1,25 @@
+#pragma once
+
+#include <solanaceae/util/span.hpp>
+
+#include <vector>
+#include <variant>
+
+struct File2I {
+	// read only
+	uint64_t _file_size {0};
+
+	const bool can_write {false};
+	const bool can_read {false};
+	// TODO: non-seekable files?
+
+	explicit File2I(bool can_write_, bool can_read_) : can_write(can_write_), can_read(can_read_) {}
+	virtual ~File2I(void) {}
+
+	virtual bool isGood(void) = 0;
+
+	// pos -1 means stream, append to last written, or read position (independent, like FILE*s)
+	virtual bool write(const ByteSpan data, int64_t pos = -1) = 0;
+	virtual std::variant<ByteSpan, std::vector<uint8_t>> read(uint64_t size, int64_t pos = -1) = 0;
+};
+