Construct the TokenMigrator contract.
This function triggers the snapshot upon which all migrations will be based.
_rate: the number of old tokens it takes to generate one new token.
_oldToken: address of the token being migrated from.
_newToken: address of the token being migrated to.
Migrates the tokenHolder’s old tokens to new tokens.
This function can only be called once per
tokenHolder. Anyone can call this method
on behalf of any other token holder since there is no disadvantage to receiving the tokens earlier.
tokenHolder: address of the token holder to migrate.