perf: a little optimization for Transactions() methods

[igorcrevar]

Changes inside Transactions() []Transaction methods:

• in advance allocate memory for returning slice of Transactions (size of slice is known)
• use map to mark invalid transactions replacing the previous loop through an invalid slice.

Additionally, a cachedTransactions slice can be introduced within each struct, allowing for immediate return from the method when the slice is not nil, if you ok with that?

[agaffney]

This looks great, and I even learned a few things (such as t.Run()). Thanks for your contribution!

[agaffney]

@igorcrevar it's probably not that helpful to cache the transactions when it's only avoiding building a small array of known size