Serialise bigint as idiomatic Filecoin form

[rvagg]

Without this it naively serialises using the underlying Rust BigInt representation:

[sign,[data,data,...]]

BigInts are going to be a footgun for these events, any time we encounter a DataCap, TokenAmount or some other BigInt wrapper since we need this explicit serde callout. I'll follow up tomorrow with a PR to the FIP-0083 doc to add some clarification around what "bigint" means for serialising this field, then hopefully future FIP authors will take note of that.

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Example field value from this event without this fix: 8201821a4876e80017 which is [1,[1215752192,23]], or specifically:

82                                                # array(2)
  01                                              #   uint(1)
  82                                              #   array(2)
    1a 4876e800                                   #     uint(1215752192)
    17                                            #     uint(23)

With this fix we get: 4600174876e800, or:

46                                                # bytes(6)
  00174876e800                                    #   "\x00\x17Hvè\x00"

Which decodes with github.com/filecoin-project/go-state-types/big.Big#UnmarshalCBOR as what I fed in to it: 100000000000.

[Stebalien]

Well, that's really annoying. Ideally we'd:

1. Move all this stuff out of fvm_shared. StoragePower doesn't belong there.
2. Modify the type such that it serializes correclty.

[Stebalien]

Code LGTM, but I didn't audit for other cases.