Fix GC safety warning

[ire4ever1190]

Currently insertID (and probably other procs) cannot be used in GC safe functions because the body parameter isn't considered GC safe

Sample from my stacktrace (using latest Nim devel version)

Warning: 'tryWithStmt' is not GC-safe as it performs an indirect call via 'body' [GcUnsafe2]
Warning: 'tryInsertID' is not GC-safe as it calls 'tryWithStmt' [GcUnsafe2]
Warning: 'insertID' is not GC-safe as it calls 'tryInsertID' [GcUnsafe2]

This just annotates the body proc as GC safe so that tryWithStmt is also considered GC safe

[ire4ever1190]

Closes #18