Refactor the internal sql interface somewhat

Experiment with refactoring the internal sql interface a bit.  My
original goal was to modify the schema and thus refactor to a state
where it would be sane to write tests for dc_sqlite_open() (and/or
however it ends up refactored) to assert schame changes before/after.

src/dc_tools.rs

@@ -1562,6 +1562,45 @@ pub fn as_str<'a>(s: *const libc::c_char) -> &'a str {
     unsafe { std::ffi::CStr::from_ptr(s).to_str().unwrap() }
 }
 
+/// Convert a C `*char` pointer to a [std::path::Path] slice.
+///
+/// This converts a `*libc::c_char` pointer to a [Path] slice.  This
+/// essentially has to convert the pointer to [std::ffi::OsStr] to do
+/// so and thus is the inverse of [OsStrExt::to_c_string].  Just like
+/// [OsStrExt::to_c_string] requires valid Unicode on Windows, this
+/// requires that the pointer contains valid UTF-8 on Windows.
+///
+/// Because this returns a reference the [Path] silce can not outlive
+/// the original pointer.
+///
+/// [Path]: std::path::Path
+#[cfg(not(target_os = "windows"))]
+pub fn as_path<'a>(s: *const libc::c_char) -> &'a std::path::Path {
+    assert!(!s.is_null(), "cannot be used on null pointers");
+    use std::os::unix::ffi::OsStrExt;
+    unsafe {
+        let c_str = std::ffi::CStr::from_ptr(s).to_bytes();
+        let os_str = std::ffi::OsStr::from_bytes(c_str);
+        std::path::Path::new(os_str)
+    }
+}
+
+// as_path() implementation for windows, documented above.
+#[cfg(target_os = "windows")]
+pub fn as_path<'a>(s: *const libc::c_char) -> &'a std::path::Path {
+    as_path_unicode(s)
+}
+
+// Implmentation for as_path() on Windows.
+//
+// Having this as a separate function means it can be tested on unix
+// too.
+#[allow(dead_code)]
+fn as_path_unicode<'a>(s: *const libc::c_char) -> &'a std::path::Path {
+    assert!(!s.is_null(), "cannot be used on null pointers");
+    std::path::Path::new(as_str(s))
+}
+
 pub fn time() -> i64 {
     SystemTime::now()
         .duration_since(SystemTime::UNIX_EPOCH)
@@ -2038,4 +2077,18 @@ mod tests {
             CStringError::InteriorNullByte
         );
     }
+
+    #[test]
+    fn test_as_path() {
+        let some_path = std::ffi::CString::new("/some/path").unwrap();
+        let ptr = some_path.as_ptr();
+        assert_eq!(as_path(ptr), std::ffi::OsString::from("/some/path"))
+    }
+
+    #[test]
+    fn test_as_path_unicode_fn() {
+        let some_path = std::ffi::CString::new("/some/path").unwrap();
+        let ptr = some_path.as_ptr();
+        assert_eq!(as_path_unicode(ptr), std::ffi::OsString::from("/some/path"))
+    }
 }