Layers with equivalence info fail to serialize to JSON due to BigInt serializing failure

[seankmartin]

See this link - the JSON state editor won't be able to open because of the following issue:

Uncaught TypeError: Do not know how to serialize a BigInt
    at JSON.stringify (<anonymous>)
    at kH.getJson (state_editor.ts:152:17)
    at kH.updateView (state_editor.ts:102:35)
    at new kH (state_editor.ts:73:10)
    at Tu.editJsonState (viewer.ts:1184:5)
    at HTMLDivElement.<anonymous> (viewer.ts:870:14)

I believe this is due to the seg IDs stored in the equivalence being BigInt type