Large code size caused by vtable initialization in `prepareVTables` function

The generated `vtable` initialization code in the [prepareVTables](https://github.com/exaloop/codon/blob/ab7387b66a7be8b1cedafc6aec10b0b402d769ff/codon/parser/visitors/typecheck/infer.cpp#L380) function is quite significant and result in increased code size and increased initialization time.

By performing a simple test, the value of `classRealizationCnt` can be increased to 1487, resulting in a significant increase in the time required for `vtable` initialization and leading to a considerable increase in code size.

Moreover, the `vtable` initialization code could prevent the optimization of some `thunk` functions that are not being used, further exacerbating the issue.