Staking precompile delegation amount getter

[4meta5]

For jira ticket 1099, which was a feature request proposed by Henry:

There is currently no way to get the current nomination amount given a nominator and a collator, through our staking precompile, and this is a commonly used method for a lot of staking related scenarios and dapps, so would be good to add support for this.

Also added a precompile method to check if the delegation is in the top delegations

[girazoki]

I see we have a typescript test for delegations, can we add these getters to the test too? Just to make sure selectors are correct

[4meta5]

I'll leave this open until another PR brings it back against master. There are too many merge conflicts to make this branch worth maintaining.

[4meta5]

Superceded by #2005