Add support to GetIntOrZero()

[lock9]

Is your feature request related to a problem? Please describe.

The following code will throw an 'Invalid Cast Exception' in runtime:

	balance := storage.Get(ctx, from).(int)
	balance += amount
	storage.Put(ctx, from, balance)

This is because it returns nil when the key is not present.

Describe the solution you'd like

I like how Neow3j solves this problem by exposing a GetIntOrZero method. As the name suggests, it will return 0 if the key is not present.

Reference:
https://github.com/neow3j/neow3j/blob/f2b243a5a01f864a3ee9b98de52458ad534a2f88/devpack/src/main/java/io/neow3j/devpack/Storage.java#L165

Describe alternatives you've considered

I'm checking for nil before using it.

Additional context

Casting will no longer be necessary with this improvement. Another change that could help, would be to allow Get calls without the context parameter.

Don't forget to add labels!

• component label: compiler
• category: enhancement

[roman-khimov]

Related to #3381, likely can even be replaced by it.