Provide variants of all interfaces with typed arguments

Most of the APIs we currently expose take `&str` arguments whose format is strictly dictated by [RFC 3501](https://tools.ietf.org/html/rfc3501). That's unfortunate, because it makes it easy for users to do things that are incorrect. Instead, we should write up typed versions of all those APIs (probably using the [builder pattern](https://rust-lang-nursery.github.io/api-guidelines/type-safety.html#c-builder)) such that none of the APIs can be misused. We *may* want to just provide builders for the most common arguments, and then continue to expose string-only APIs for the more esoteric use-cases.

This should help with #74, and would be a good way to deal with #60.

See the original issue here: https://github.com/mattnenterprise/rust-imap/issues/99

Start by inventorying the exposed APIs that accept &str values governed by RFC 3501, then identify which arguments are common enough for typed builders. Review Rust's type-safety builder-pattern guidance and the related issues #74 and #60. Done means the API offers typed variants that prevent misuse while retaining string-only access where necessary.

Written by the indexing model from the issue text.