Treat transaction args as opaque data

[aboodman]

Currently transaction arguments are decoded in both repc and replicache-sdk-js in various locations. This is both expensive and also prevents us from easily doing useful sanity checks.

[aboodman]

See https://github.com/rocicorp/repc/pull/153/commits/9e73ae592d2980a7c9f360fc1d9625b84b891564 for previous sanity check that should be re-instated with this bug.

[aboodman]

See #139

[phritz]

args in opentransaciton become a string https://github.com/rocicorp/repc/blob/a5ee7e5caeebdd41205046356853b22dfd0aed56/src/embed/types.rs#L13

[phritz]

this forces one of three choices:

1. move to something not json at the boundaries with customers (prolly not tenable rn)
2. base64 encode values
3. or use strings

[phritz]

heh:

• opentransaction args becomes utf8 string, gets stored opaquely
• we still probably need Any to encode the string when calling batchpush