* feat: properly implement math ops for bigint
* feat: remove bigint x string and bigint x num ops
every op now requires explicit conversion
* fix: remove dead code
Jsonnet spec defines special behavior for `super.field` and
`field in super` expressions, while in jrsonnet it is implemented by
standalone-super concept, with optimizations in case of `super.field`.
However, standalone-super is conflicting with `field in super` behavior,
so special handling is needed for this expression.