Make the Store API more type-safe

Most functions now take a StorePath argument rather than a Path (which
is just an alias for std::string). The StorePath constructor ensures
that the path is syntactically correct (i.e. it looks like
<store-dir>/<base32-hash>-<name>). Similarly, functions like
buildPaths() now take a StorePathWithOutputs, rather than abusing Path
by adding a '!<outputs>' suffix.

Note that the StorePath type is implemented in Rust. This involves
some hackery to allow Rust values to be used directly in C++, via a
helper type whose destructor calls the Rust type's drop()
function. The main issue is the dynamic nature of C++ move semantics:
after we have moved a Rust value, we should not call the drop function
on the original value. So when we move a value, we set the original
value to bitwise zero, and the destructor only calls drop() if the
value is not bitwise zero. This should be sufficient for most types.

Also lots of minor cleanups to the C++ API to make it more modern
(e.g. using std::optional and std::string_view in some places).

src/libutil/util.hh

@@ -62,7 +62,7 @@ Path dirOf(const Path & path);
 
 /* Return the base name of the given canonical path, i.e., everything
    following the final `/'. */
-string baseNameOf(const Path & path);
+std::string_view baseNameOf(std::string_view path);
 
 /* Check whether 'path' is a descendant of 'dir'. */
 bool isInDir(const Path & path, const Path & dir);
@@ -407,7 +407,7 @@ bool hasPrefix(const string & s, const string & prefix);
 
 
 /* Return true iff `s' ends in `suffix'. */
-bool hasSuffix(const string & s, const string & suffix);
+bool hasSuffix(std::string_view s, std::string_view suffix);
 
 
 /* Convert a string to lower case. */
@@ -450,10 +450,10 @@ string base64Decode(const string & s);
 /* Get a value for the specified key from an associate container, or a
    default value if the key doesn't exist. */
 template <class T>
-string get(const T & map, const string & key, const string & def = "")
+std::optional<std::string> get(const T & map, const std::string & key)
 {
     auto i = map.find(key);
-    return i == map.end() ? def : i->second;
+    return i == map.end() ? std::optional<std::string>() : i->second;
 }