Streamlines the Scanner Tests. (#25)

Previously, the `Scanner` tests required a lot of manual position
expectations which was tedious to write.  This replaces that with a test
harness that can do the math from chunks of input that the test writer
defines as whitespace or token boundaries.  This also adds the macro
`scanner_test_case!` which makes it so that one can use the harness with
a friendly syntax. The old tests are ported to the new macro.

Also:
* Adds assertions to the internal `LineAndColumn::at`.
* Adds convenience constructor around `Content`.

Resolves #21.

Author: almann

partiql-parser/src/result.rs

@@ -24,6 +24,8 @@ impl LineAndColumn {
     /// Constructs at [`LineAndColumn`] without verifying 1-position invariant.
     #[inline]
     pub(crate) fn at(line: usize, column: usize) -> Self {
+        assert_ne!(0, line);
+        assert_ne!(0, column);
         Self(line, column)
     }