add mem.readVarInt, fix InStream.readVarInt, fix stack traces

fixes a regression from b883bc8

std/debug/index.zig

@@ -1200,7 +1200,7 @@ const Constant = struct {
     fn asUnsignedLe(self: *const Constant) !u64 {
         if (self.payload.len > @sizeOf(u64)) return error.InvalidDebugInfo;
         if (self.signed) return error.InvalidDebugInfo;
-        return mem.readIntSliceLittle(u64, self.payload);
+        return mem.readVarInt(u64, self.payload, builtin.Endian.Little);
     }
 };
 

std/io.zig

@@ -183,11 +183,12 @@ pub fn InStream(comptime ReadError: type) type {
             return mem.readIntSlice(T, bytes, endian);
         }
 
-        pub fn readVarInt(self: *Self, comptime T: type, endian: builtin.Endian, size: usize) !T {
-            assert(size <= @sizeOf(T));
-            var bytes: [@sizeOf(T)]u8 = undefined;
-            try self.readNoEof(bytes[0..size]);
-            return mem.readIntSlice(T, bytes, endian);
+        pub fn readVarInt(self: *Self, comptime ReturnType: type, endian: builtin.Endian, size: usize) !ReturnType {
+            assert(size <= @sizeOf(ReturnType));
+            var bytes_buf: [@sizeOf(ReturnType)]u8 = undefined;
+            const bytes = bytes_buf[0..size];
+            try self.readNoEof(bytes);
+            return mem.readVarInt(ReturnType, bytes, endian);
         }
 
         pub fn skipBytes(self: *Self, num_bytes: usize) !void {

std/mem.zig

@@ -407,6 +407,27 @@ test "mem.indexOf" {
     assert(lastIndexOfScalar(u8, "boo", 'o').? == 2);
 }
 
+/// Reads an integer from memory with size equal to bytes.len.
+/// T specifies the return type, which must be large enough to store
+/// the result.
+pub fn readVarInt(comptime ReturnType: type, bytes: []const u8, endian: builtin.Endian) ReturnType {
+    var result: ReturnType = 0;
+    switch (endian) {
+        builtin.Endian.Big => {
+            for (bytes) |b| {
+                result = (result << 8) | b;
+            }
+        },
+        builtin.Endian.Little => {
+            const ShiftType = math.Log2Int(ReturnType);
+            for (bytes) |b, index| {
+                result = result | (ReturnType(b) << @intCast(ShiftType, index * 8));
+            }
+        },
+    }
+    return result;
+}
+
 /// Reads an integer from memory with bit count specified by T.
 /// The bit count of T must be evenly divisible by 8.
 /// This function cannot fail and cannot cause undefined behavior.