fix(gnovm): assignment operators require 1 expression on both sides

[thehowl]

This PR fixes a bug in the GnoVM which executes incorrect statements like s, ok <<= m["a"].

The following code currently executes on gno:

package main

func main() {
	m := map[string]int{"a": 1}
	var s int
	var ok bool
	s, ok <<= m["a"]
	println(s, ok)
}

This PR matches the behaviour on assignment statements to the Go specification:

An assignment operation x op= y where op is a binary arithmetic operator is equivalent to x = x op (y) but evaluates x only once. The op= construct is a single token. In assignment operations, both the left- and right-hand expression lists must contain exactly one single-valued expression, and the left-hand expression must not be the blank identifier.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 47.78%. Comparing base (3d1d26c) to head (fd022c6).
Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1943   +/-   ##
=======================================
  Coverage   47.78%   47.78%           
=======================================
  Files         393      393           
  Lines       61608    61602    -6     
=======================================
  Hits        29437    29437           
+ Misses      29701    29695    -6     
  Partials     2470     2470           

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[thehowl]

I double-checked, and this same check is supposed to be done for all assignment operators (like +=), according to the Go spec.

I've made the changes, just want to get another approval before merging this.