Add Matchtime captures to peg (Equivalent to LPegs lpeg.Cmt).

This allows that pattern to call an external function to
check if some text should match or not. This allows for
matching any possible language a computer can recognize.

test/suite3.janet

@@ -53,7 +53,7 @@
 
 (assert (= var-b "hello") "regression 1")
 
-# Scan number 
+# Scan number
 
 (assert (= 1 (scan-number "1")) "scan-number 1")
 (assert (= -1 (scan-number "-1")) "scan-number -1")
@@ -189,13 +189,21 @@
 
 (peg/match pegleg "abc,abc")
 
+# Bad Grammars
+
+(assert-error "peg/compile error 1" (peg/compile nil))
+(assert-error "peg/compile error 2" (peg/compile @{}))
+(assert-error "peg/compile error 3" (peg/compile '{:a "abc" :b "def"}))
+(assert-error "peg/compile error 4" (peg/compile '(blarg "abc")))
+(assert-error "peg/compile error 5" (peg/compile '(1 2 3)))
+
 # IP address
 
 (def ip-address
   '{:d (range "09")
     :0-4 (range "04")
     :0-5 (range "05")
-    :byte (+ 
+    :byte (+
             (* "25" :0-5)
             (* "2" :0-4 :d)
             (* "1" :d :d)
@@ -261,4 +269,13 @@
 
 (check-deep '(+ (* (capture "c") "d") "ce") "ce" @[])
 
+# Matchtime capture
+
+(def scanner (peg/compile ~(cmt (some 1) ,scan-number)))
+
+(check-deep scanner "123" @[123])
+(check-deep scanner "0x86" @[0x86])
+(check-deep scanner "-1.3e-7" @[-1.3e-7])
+(check-deep scanner "123A" nil)
+
 (end-suite)