FEAT/CHANGE: added support for find on map!

This feature was introduced in Red language so it is good to be compatible.
Example:
```
>> find #("a" 1 "b" "c") "A"
== "a"

>> find #("a" 1 "b" "c") 1
== none

>> find/case #("a" 1 "b" "c") "A"
== none
```

Author: Oldes

src/boot/actions.r

@@ -193,7 +193,7 @@ pick: action [
 
 find: action [
 	{Searches for a value; for series returns where found, else none.}
-	series [series! gob! port! bitset! typeset! object! none!]
+	series [series! gob! port! bitset! typeset! object! map! none!]
 	value [any-type!]
 	/part {Limits the search to a given length or position}
 	length [number! series! pair!]

src/core/t-map.c

@@ -488,9 +488,10 @@
 
 	case A_PICK:		// same as SELECT for MAP! datatype
 	case A_SELECT:
+	case A_FIND:
 		n = Find_Entry(series, arg, 0, Find_Refines(ds, AM_SELECT_CASE) ? AM_FIND_CASE : 0);
 		if (!n) return R_NONE;
-		*D_RET = *VAL_BLK_SKIP(val, ((n-1)*2)+1);
+		*D_RET = *VAL_BLK_SKIP(val, ((n-1)*2)+((action == A_FIND)?0:1));
 		break;
 
 	case A_INSERT:

src/tests/units/map-test.r3

@@ -79,6 +79,35 @@ Rebol [
 ===end-group===
 
 
+===start-group=== "find"
+
+	--test-- "map-find-1"
+		mf1-m: #(a: none b: 1 c: 2)
+		--assert 'a		= find mf1-m 'a
+		--assert 'b		= find mf1-m 'b
+		--assert 'c		= find mf1-m 'c
+		--assert none	= find mf1-m 'd
+		
+	--test-- "map-find-2"
+		mf2-m: #(a: 1 b: 2 c: 3)
+		mf2-m/a: 'none
+		mf2-m/b: none
+		--assert 'a = find mf2-m 'a
+		--assert 'b = find mf2-m 'b
+		--assert 'c = find mf2-m 'c
+
+	--test-- "map-find-3"
+		mf3-m: #(aB: 1 ab: 2 AB: 3)
+		--assert 'ab = find/case mf3-m 'ab
+		--assert none = find/case mf3-m 'Ab
+
+	--test-- "map-find-4"
+		mf4-m: make map! [b 1]
+		--assert 'b	= find mf4-m first [b:]
+		--assert 'b	= find mf4-m first [b:]
+===end-group===
+
+
 ===start-group=== "map issues"
 	;@@ https://github.com/Oldes/Rebol-issues/issues/770
 	--test-- "map-issue-770"