Use method namespace icrc1_ instead of icrc2_

[benjizhai]

I think all standards extending the ICRC-1 standard shall use the namespace icrc1_* for their methods.
Compare the following code snippet when multiple standards are implemented:
icrc2_transfer_from icrc3_get_transactions icrc4_batch_fransfer icrc1_get_metadata
vs
icrc1_transfer_from icrc1_get_transactions icrc1_batch_fransfer icrc1_get_metadata
I would say that the former is very confusing as the developer using the token needs to remember which methods belong to which ICRC number. Where in the latter case as long as the following ICRCs don't have overlapping names (which we can control as part of the ICRC process) it's very easy to use.