proc: increase maximum string length when loading string for binary ops (#1620)

Increases the maximum string length from 64 to 1MB when loading strings
for a binary operator, also delays the loading until it's necessary.

This ensures that comparison between strings will always succeed in
reasonable situations.

Fixes #1615

pkg/proc/variables.go

@@ -163,6 +163,7 @@ type LoadConfig struct {
 
 var loadSingleValue = LoadConfig{false, 0, 64, 0, 0, 0}
 var loadFullValue = LoadConfig{true, 1, 64, 64, -1, 0}
+var loadFullValueLongerStrings = LoadConfig{true, 1, 1024 * 1024, 64, -1, 0}
 
 // G status, from: src/runtime/runtime2.go
 const (