Add comparison date function in the SC library

[samuelmanzanera]

We should add function to compare datetime

Because this is not valid:

condition transaction: [
  timestamp: < unix_timestamp
]

we could indeed as something like:

condition transaction: [
  timestamp: Time.compare(unix_timestamp) == "lt"
  # OR
  timestamp: Time.diff(unix_timestamp) > 0
]

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Epic: #608