add: rc20 compatibility

Some changes were backward-incompatible, and the firehose
@initialize and @finalize scripts must now declare a metavariables
region.
See https://github.com/coccinelle/coccinelle/commit/eee0552c6a8c61222724744b931ade3100d0770f
Merge pull request #4 from matthieucan/fix/contrib-firehose-rc20-compatibility

add: rc20 compatibility