Refactor HandlerOutput and HandlerResult

[plafer]

HandlerOutput and HandlerResult should be modeled roughly as:

struct HandlerResult<T> {
    // We want to return logs even if the processing fails
    logs: Vec<String>,
    result: Result<HandlerOutput<T>, Error>
}

struct HandlerOutput<T> {
    // previously `result`, which "collides" with Rust's `Result`
    value: T,
    events: Vec<IbcEvent>,
}

HandlerOutputBuilder would also be renamed to HandlerResultBuilder, and build a HandlerResult. with_result() could be split into with_value(T), and without_value() (to avoid the ugly with_result(()) when there's no result). Hopefully we could get rid of merge() and merge_output() as well.

@hu55a1n1 and @romac contributed to these ideas.

[plafer]

Related: #23

[plafer]

This API is no longer used since #279. It will be completely removed after #442.