make tagset's const char* ctor a static creation function

libcorpus2/tagset.cpp

@@ -73,12 +73,11 @@ Tagset::Tagset()
 {
 }
 
-Tagset::Tagset(const char *s)
-	: id_(++next_id_)
+Tagset Tagset::from_data(const char *s)
 {
 	std::stringstream ss;
 	ss << s;
-	*this = TagsetParser::load_ini(ss);
+	return TagsetParser::load_ini(ss);
 }
 
 std::string Tagset::id_string() const

libcorpus2/tagset.h

@@ -99,10 +99,10 @@ public:
 	Tagset();
 
 	/**
-	 * Tagset convenience constructor, parse a string as if it were the
+	 * Tagset convenience creation function, parse a string as if it were the
 	 * contents of a tagset ini file
 	 */
-	explicit Tagset(const char*);
+	static Tagset from_data(const char*);
 
 	/**
 	 * Tag parsing -- functional version, whole tag string.