compress.q compatibility with KDB 3.6+

[SMLaursen]

We're having issues running compress.q on data with nested lists on KDB 3.6(+)

Specifically the issue appear to be caused by the "if[78 <= type get.... " check on line 172 since
"Mapped list type (77h) deprecates old mapped nested types (77h+t). 77h+t files are read-only."

We've experimented with using 77 instead which appears to work across the different kdb versions