Add functions packages support (#419)

* Add packaged functions support

* Add tests for math.floor func

* Export FunctionsTable

* Extract func stuff into its own package

* Rename stuff

* Fix tests

* Move doc package to cmd/genji

* Adjust naming, typos

internal/expr/functions/math_test.go

@@ -0,0 +1,12 @@
+package functions_test
+
+import (
+	"path/filepath"
+	"testing"
+
+	"github.com/genjidb/genji/internal/testutil"
+)
+
+func TestMathFunctions(t *testing.T) {
+	testutil.ExprRunner(t, filepath.Join("testdata", "math_functions.sql"))
+}