Add SHA1 algorithm implementation. (#32)

Largely based on https://github.com/vog/sha1 with some customizations and
optimizations.

src/main/native/include/support/sha1.h

@@ -0,0 +1,55 @@
+/*
+    sha1.hpp - header of
+
+    ============
+    SHA-1 in C++
+    ============
+
+    100% Public Domain.
+
+    Original C Code
+        -- Steve Reid <steve@edmweb.com>
+    Small changes to fit into bglibs
+        -- Bruce Guenter <bruce@untroubled.org>
+    Translation to simpler C++ Code
+        -- Volker Grabsch <vog@notjusthosting.com>
+    Safety fixes
+        -- Eugene Hopkinson <slowriot at voxelstorm dot com>
+*/
+
+#ifndef SHA1_HPP
+#define SHA1_HPP
+
+
+#include <cstdint>
+#include <string>
+#include "llvm/StringRef.h"
+
+namespace llvm {
+template <typename T> class SmallVectorImpl;
+}
+
+namespace wpi {
+
+class raw_istream;
+
+class SHA1
+{
+public:
+    SHA1();
+    void Update(llvm::StringRef s);
+    void Update(raw_istream &is);
+    std::string Final();
+    llvm::StringRef Final(llvm::SmallVectorImpl<char>& buf);
+    static std::string FromFile(llvm::StringRef filename);
+
+private:
+    uint32_t digest[5];
+    unsigned char buffer[64];
+    size_t buf_size;
+    uint64_t transforms;
+};
+
+}  // namespace wpi
+
+#endif /* SHA1_HPP */