Avoid double-lookup dictionaries: .ContainsKey key followed by indexer call .[key]

Avoid double-lookup dictionaries:

.ContainsKey key followed by .[key] should be replaced with .TryGetValue x

for example:
```fsharp
match xs.ContainsKey x with
| true -> ... xs.[x] ...
| false -> ///...
```

should be replaced with
```fsharp
match xs.TryGetValue x with
| true, xVal -> ... xVal ...
| false, _ -> ///...
```
