crucible: add missing template specializations of pwrite helper functions

I got a little too enthusiastic when redacting the code, and removed some
overloaded functions bees was using.  C++ silently found replacements,
and the result was a bug that prevented any data from being persisted
from the hash table.

Fixes: https://github.com/Zygo/bees/issues/7

include/crucible/fd.h

@@ -120,6 +120,9 @@ namespace crucible {
 	template<> void pread_or_die<string>(int fd, string& str, off_t offset);
 	template<> void pread_or_die<vector<char>>(int fd, vector<char>& str, off_t offset);
 	template<> void pread_or_die<vector<uint8_t>>(int fd, vector<uint8_t>& str, off_t offset);
+	template<> void pwrite_or_die<string>(int fd, const string& str, off_t offset);
+	template<> void pwrite_or_die<vector<char>>(int fd, const vector<char>& str, off_t offset);
+	template<> void pwrite_or_die<vector<uint8_t>>(int fd, const vector<uint8_t>& str, off_t offset);
 
 	// A different approach to reading a simple string
 	string read_string(int fd, size_t size);

lib/fd.cc

@@ -426,6 +426,27 @@ namespace crucible {
 		return pread_or_die(fd, text.data(), text.size(), offset);
 	}
 
+	template<>
+	void
+	pwrite_or_die<vector<uint8_t>>(int fd, const vector<uint8_t> &text, off_t offset)
+	{
+		return pwrite_or_die(fd, text.data(), text.size(), offset);
+	}
+
+	template<>
+	void
+	pwrite_or_die<vector<char>>(int fd, const vector<char> &text, off_t offset)
+	{
+		return pwrite_or_die(fd, text.data(), text.size(), offset);
+	}
+
+	template<>
+	void
+	pwrite_or_die<string>(int fd, const string &text, off_t offset)
+	{
+		return pwrite_or_die(fd, text.data(), text.size(), offset);
+	}
+
 	Stat::Stat()
 	{
 		memset_zero<stat>(this);