Parser Error for Reserved Keywords

[ArquintL]

While we should namespace Gobra's keywords in the midterm future (cf. #118), the parser should provide better error message for the time being.
E.g., the following bitLen function from the Go standard library (crypto/internal/fips140/bigmod) produces a "no viable alternative at input 'len :=' error message, which is fine. However, having (not even verifying!) the very same function in a larger file, produces the following error at a completely unrelated portion of the file

unexpected ==, expecting arguments
        assert reveal n.AnnouncedLen() == 0

// Copyright 2021 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package test

func bitLen(n uint) int {
	len := 0
	// We assume, here and elsewhere, that comparison to zero is constant time
	// with respect to different non-zero values.
	for n != 0 {
		len++
		n >>= 1
	}
	return len
}