implemented a[^1] notation

tests/array/troof1.nim

@@ -0,0 +1,36 @@
+discard """
+  output: '''@[2, 3, 4]321
+9.0 4.0
+(a: 1.0, b: 2.0, c: 8.0)2.0'''
+"""
+
+proc foo[T](x, y: T): T = x
+
+var a = @[1, 2, 3, 4]
+var b: array[3, array[2, float]] = [[1.0,2], [3.0,4], [8.0,9]]
+echo a[1.. ^1], a[^2], a[^3], a[^4]
+echo b[^1][^1], " ", (b[^2]).foo(b[^1])[^1]
+
+type
+  MyArray = object
+    a, b, c: float
+
+var
+  ma = MyArray(a: 1.0, b: 2.0, c: 3.0)
+
+proc len(x: MyArray): int = 3
+
+proc `[]=`(x: var MyArray; idx: range[0..2]; val: float) =
+  case idx
+  of 0: x.a = val
+  of 1: x.b = val
+  of 2: x.c = val
+
+proc `[]`(x: var MyArray; idx: range[0..2]): float =
+  case idx
+  of 0: result = x.a
+  of 1: result = x.b
+  of 2: result = x.c
+
+ma[^1] = 8.0
+echo ma, ma[^2]

tests/array/troof2.nim

@@ -0,0 +1,10 @@
+discard """
+  errormsg: "invalid context for '^' as 'foo()' has side effects"
+  line: "9"
+"""
+
+proc foo(): seq[int] =
+  echo "ha"
+
+let f = foo()[^1]
+

tests/array/troof3.nim

@@ -0,0 +1,8 @@
+discard """
+  errormsg: "invalid context for '^' as len!=high+1 for 'a'"
+  line: "8"
+"""
+
+var a: array[1..3, string]
+
+echo a[^1]

tests/array/troof4.nim

@@ -0,0 +1,37 @@
+discard """
+  errormsg: "no surrounding array access context for '^'"
+  line: "37"
+"""
+
+proc foo[T](x, y: T): T = x
+
+var a = @[1, 2, 3, 4]
+var b: array[3, array[2, float]] = [[1.0,2], [3.0,4], [8.0,9]]
+echo a[1.. ^1], a[^2], a[^3], a[^4]
+echo b[^1][^1], " ", (b[^2]).foo(b[^1])[^1]
+
+type
+  MyArray = object
+    a, b, c: float
+
+var
+  ma = MyArray(a: 1.0, b: 2.0, c: 3.0)
+
+proc len(x: MyArray): int = 3
+
+proc `[]=`(x: var MyArray; idx: range[0..2]; val: float) =
+  case idx
+  of 0: x.a = val
+  of 1: x.b = val
+  of 2: x.c = val
+
+proc `[]`(x: var MyArray; idx: range[0..2]): float =
+  case idx
+  of 0: result = x.a
+  of 1: result = x.b
+  of 2: result = x.c
+
+ma[^1] = 8.0
+echo ma, ma[^2]
+
+echo(^1)