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forp: change indexing to verilog-like semantics

--- a/sys/man/1/forp

+++ b/sys/man/1/forp

@@ -89,8 +89,9 @@

 The valid operators are listed below, in decreasing precedence. Note that logical operations treat all non-zero values as 1, whereas bitwise operators operate on all bits independently.

 .TP "\w'\fL<\fR, \fL<=\fR, \fL>\fR, \fL>=\fR  'u"

 \fL[]\fR

-Array indexing. The syntax is \fIvar\fL[\fIidx\fL:\fIn\fR] to address \fIn\fR bits with the least-significant bit at \fIidx\fR.

-Omiting \fL:\fIn\fR addresses a single bit.

+Array indexing. The syntax is \fIvar\fL[\fIa\fL:\fIb\fR], with \fIa\fR denoting the MSB and \fIb\fR denoting the LSB.

+Omiting \fL:\fIb\fR addresses a single bit.

+The result is always treated as unsigned.

 .TP

 \fL!\fR, \fL~\fR, \fL+\fR, \fL-\fR

 (Unary operators) Logical and bitwise "not", unary plus (no-op), arithmetic negation. Because of promotion, \fL~\fR and \fL-\fR operate beyond the width of variables.

--- a/sys/src/cmd/forp/cvt.c

+++ b/sys/src/cmd/forp/cvt.c

@@ -251,9 +251,9 @@

 {

 	int i, j, k, s;

-	j = mptoi(n2->num);

-	if(n3 == nil) k = j;

-	else k = mptoi(n3->num);

+	k = mptoi(n2->num);

+	if(n3 == nil) j = k;

+	else j = mptoi(n3->num);

 	if(j > k){

 		nodevars(rn, 1);

 		return;

@@ -492,7 +492,7 @@

 	case ASTIDX:

 		if(n->n2->type != ASTNUM || n->n3 != nil && n->n3->type != ASTNUM)

 			error(n, "non-constant in indexing expression");

-		convert(n->n1, (n->n3 != nil ? mptoi(n->n3->num) : mptoi(n->n2->num)) + 1);

+		convert(n->n1, n->n3 != nil ? mptoi(n->n3->num) - mptoi(n->n2->num) + 1 : 1);

 		opidx(n, n->n1, n->n2, n->n3);

 		break;

 	case ASTTERN: