Implement value typing, null support via value attributes

For example:

v = [null]
v = [string] abc
v += ABC # abcABC

3 files changed, 32 insertions, 0 deletions

diff --git a/tests/variable/null/buildfile b/tests/variable/null/buildfile
new file mode 100644
index 0000000..3fa1a9e
--- /dev/null
+++ b/tests/variable/null/buildfile
@@ -0,0 +1,22 @@
+#v = [null=junk]  # error: unexpected value for attribute null: junk
+#v = [null] junk  # error: empty null value expected instead of 'junk'
+
+print $v0 # Undefined.
+
+v1 = [null]
+print $v1
+
+v2 = x
+v2 = [null]
+print $v2
+
+v3a = [null]
+v3b = $v3a
+print $v3b
+v3b = ($v3a)
+print $v3b
+print ($v3b)
+
+print [null]
+
+./:
diff --git a/tests/variable/null/test.out b/tests/variable/null/test.out
new file mode 100644
index 0000000..5119a71
--- /dev/null
+++ b/tests/variable/null/test.out
@@ -0,0 +1,7 @@
+[null]
+[null]
+[null]
+[null]
+[null]
+[null]
+[null]
diff --git a/tests/variable/null/test.sh b/tests/variable/null/test.sh
new file mode 100755
index 0000000..afcb3bd
--- /dev/null
+++ b/tests/variable/null/test.sh
@@ -0,0 +1,3 @@
+#!/bin/sh
+
+b -q | diff -u test.out -