Support instant sealing

[ermalkaleci]

Add support instant sealing which is useful for testing contracts.

NOTE: Although I creating a MockProvider to make waffle works, I still have to add a little delay (0.1s) between transaction to make it work but it's better then 6s

[JoshOrndorff]

Looking good IMO @ermalkaleci Two more thoughts I'll share:

1. I wonder if "Timestamp" should be in the name of your struct. like maybe it should be MockTimestampInherentDataProvider or something shorter, but with Timestamp included. I don't feel strongly, just sharing the idea.

2. This is my first time encountering static mut but I did read over https://doc.rust-lang.org/reference/items/static-items.html . I wonder if using a field on the struct is a better idea and can avoid the unsafe block. Something like this:

pub struct MockTimestampInherentDataProvider {
  current_timestamp: u64,
}

[JoshOrndorff]

Well, I guess my idea #2 above won't work because we can't mutate the instance. Maybe an alternative approach would be to read the timestamp inherent of the previous block? I'm just thinking of ways to remove the unsafe. But if @sorpaas is okay with it, I am too.

[ermalkaleci]

@JoshOrndorff I think unsafe is fine since this is going to be used only in testing mode. Anyway we can also do

thread_local! {
	static CURRENT_TIMESTAMP: RefCell<u64> = RefCell::new(0);
}

and get rid-off unsafe

[sorpaas]

LGTM! Just a minor nit.