Compact serialisation of IfaceAppArgs

In #24563, we identified that IfaceAppArgs serialisation tags each
cons cell element with a discriminator byte. These bytes add up
quickly, blowing up interface files considerably when
'-fwrite-if-simplified-core' is enabled.

We compact the serialisation by writing out the length of
'IfaceAppArgs', followed by serialising the elements directly without
any discriminator byte.

This improvement can decrease the size of some interface files by up
to 35%.