Make --show-iface more human readable

Right now --show-iface is not particularly interpretable, even if you are looking at the Outputable instance for ModIface. The problem seems to be the output for mi_deps, mi_usages, mi_decls, mi_insts, mi_fam_insts and mi_rules is not clearly delimited, making it unclear (for example) what import -/ base:Prelude HASH refers to unless you already happen to know that this must be from mi_usages.

It should be relatively easy to improve this, but I want to make sure people who rely on this output aren't blindsided by the changes.

Trac metadata

Trac field	Value
Version	7.9
Type	Task
TypeOfFailure	OtherFailure
Priority	low
Resolution	Unresolved
Component	Compiler
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