Replace 'req' with 'http-client'

[stevladimir]

http-client is kinda a standard for low-level HTTP. req adds some convenience layer over it, but we mostly don't use that. Moreover, its model rather makes things less transparent and more fragile. E.g. we need direct control over HTTP status codes to handle non-success responses, what req obscures (see this thread). servant could also be an option here, but it is too heavy-weight for this use case.
ConnexpayM is also removed. Instead we pass (Config) and return (Response e a) things explicitly. This allows, e.g. to have separate error types for different endpoints. Previously we had single PaymentFailure for all calls, but obviously errors like CVVFailed could not be thrown from, say, void endpoint.
Removal of req also reduces dependency footprint, what is good for a public library.

Other changes done on the way:

• Dropped "bucks" dependency
  We use neither of its features and all of it is replaced with trivial USD newtype.
• Log accurately request and response (full body regardless of the size)
  In a public library we should probably use something more flexible than Text -> IO () as a logger, but that is a separate story
• Build request internals around ToJSON instead of [Pair]
• Consistent masking of request bodies
• Factor out dynamic part of the environment (Env) making it part of the request
  Now it is easy to set per-request logger to attach request-specific context to the Connexpay logs