Move library crates under 'lib/'.

Give these crates each a more standard directory layout with sources in
a 'src' sub-sirectory and Cargo.toml in the top lib/foo directory.

Add license and description fields to each.

The build script for the cretonne crate now lives in
'lib/cretonne/build.rs' separating it from the normal library sources
under 'lib/cretonne/src'.

lib/filecheck/src/variable.rs

@@ -0,0 +1,60 @@
+use std::borrow::Cow;
+
+/// A variable name is one or more ASCII alphanumerical characters, including underscore.
+/// Note that numerical variable names like `$45` are allowed too.
+///
+/// Try to parse a variable name from the begining of `s`.
+/// Return the index of the character following the varname.
+/// This returns 0 if `s` doesn't have a prefix that is a variable name.
+pub fn varname_prefix(s: &str) -> usize {
+    for (idx, ch) in s.char_indices() {
+        match ch {
+            'a'...'z' | 'A'...'Z' | '0'...'9' | '_' => {}
+            _ => return idx,
+        }
+    }
+    s.len()
+}
+
+/// A variable can contain either a regular expression or plain text.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum Value<'a> {
+    Text(Cow<'a, str>),
+    Regex(Cow<'a, str>),
+}
+
+/// Resolve variables by name.
+pub trait VariableMap {
+    /// Get the value of the variable `varname`, or return `None` for an unknown variable name.
+    fn lookup(&self, varname: &str) -> Option<Value>;
+}
+
+impl VariableMap for () {
+    fn lookup(&self, _: &str) -> Option<Value> {
+        None
+    }
+}
+
+/// An empty variable map.
+pub const NO_VARIABLES: &'static VariableMap = &();
+
+#[cfg(test)]
+mod tests {
+    #[test]
+    fn varname() {
+        use super::varname_prefix;
+
+        assert_eq!(varname_prefix(""), 0);
+        assert_eq!(varname_prefix("\0"), 0);
+        assert_eq!(varname_prefix("_"), 1);
+        assert_eq!(varname_prefix("0"), 1);
+        assert_eq!(varname_prefix("01"), 2);
+        assert_eq!(varname_prefix("b"), 1);
+        assert_eq!(varname_prefix("C"), 1);
+        assert_eq!(varname_prefix("."), 0);
+        assert_eq!(varname_prefix(".s"), 0);
+        assert_eq!(varname_prefix("0."), 1);
+        assert_eq!(varname_prefix("01="), 2);
+        assert_eq!(varname_prefix("0a)"), 2);
+    }
+}