Added ProcessStream to for running processes (on Linux) and streaming their output.

Added map() function for creating a mapping iterator.
Added mijin::pipe types for map() and join().

source/mijin/io/process.hpp

@@ -0,0 +1,47 @@
+#pragma once
+
+#if !defined(MIJIN_IO_PROCESS_HPP_INCLUDED)
+#define MIJIN_IO_PROCESS_HPP_INCLUDED 1
+
+#include <vector>
+#include "./stream.hpp"
+
+namespace mijin
+{
+class ProcessStream : public Stream
+{
+private:
+    std::FILE* handle = nullptr;
+    FileOpenMode mode;
+    int bufferedChar; // for isAtEnd
+public:
+    ~ProcessStream() override;
+
+    StreamError open(const char* command, FileOpenMode mode_);
+    inline StreamError open(const std::string& command, FileOpenMode mode_) {
+        return open(command.c_str(), mode_);
+    }
+    inline StreamError open(const std::vector<std::string>& args, FileOpenMode mode_);
+    int close();
+    [[nodiscard]] inline bool isOpen() const { return handle != nullptr; }
+
+    // Stream overrides
+    StreamError readRaw(std::span<std::uint8_t> buffer, bool partial = false, std::size_t* outBytesRead = nullptr) override;
+    StreamError writeRaw(std::span<const std::uint8_t> buffer) override;
+    std::size_t tell() override;
+    StreamError seek(std::intptr_t pos, SeekMode seekMode = SeekMode::ABSOLUTE) override;
+    void flush() override;
+    bool isAtEnd() override;
+    StreamFeatures getFeatures() override;
+};
+
+[[nodiscard]] std::string shellEscape(const std::string& arg) noexcept;
+[[nodiscard]] std::string makeShellCommand(const std::vector<std::string>& args) noexcept;
+
+StreamError ProcessStream::open(const std::vector<std::string>& args, FileOpenMode mode_)
+{
+    return open(makeShellCommand(args), mode_);
+}
+}
+
+#endif // MIJIN_IO_PROCESS_HPP_INCLUDED