Add support for evaluation context

For now it acts as just the value mode that can be enabled anywhere
variable expansion is supported, for example:

(foo=bar):

And the primary use currently is to enable/test quoted and indirect
variable expansion:

"foo bar" = FOO BAR
print $"foo bar"    # Invalid.
print $("foo bar")  # Yeah, baby.

foo = FOO
FOO = foo
print $($foo)

Not that you should do something like this...

3 files changed, 20 insertions, 0 deletions

diff --git a/tests/eval/buildfile b/tests/eval/buildfile
new file mode 100644
index 0000000..c658d3b
--- /dev/null
+++ b/tests/eval/buildfile
@@ -0,0 +1,13 @@
+(./):
+
+# Invalid.
+#
+#(foo
+#(foo #comment
+
+print ()
+print ((foo)(bar))
+print ((foo)   (bar))
+
+print (foo\
+bar)
diff --git a/tests/eval/test.out b/tests/eval/test.out
new file mode 100644
index 0000000..5885c7d
--- /dev/null
+++ b/tests/eval/test.out
@@ -0,0 +1,4 @@
+
+foobar
+foo bar
+foobar
diff --git a/tests/eval/test.sh b/tests/eval/test.sh
new file mode 100755
index 0000000..b898b3c
--- /dev/null
+++ b/tests/eval/test.sh
@@ -0,0 +1,3 @@
+#!/bin/sh
+
+valgrind -q b -q | diff -u test.out -