types: forward-declare multiprecision_int
types: forward-declare multiprecision_int

This reduces the number of translation units that depend on
boost/multiprecision from 354 to 30, and reduces the size of
database.i (as an example) from 406160 to 382933 (smaller
files will benefit more, relatively).

Ref #1

avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla