The replace_headers method had to be moved since the async_impl module
is not compiled while compiling to wasm. This caused the replace_headers
method to be unavailable. fast_random had to be excluded from the wasm
build to prevent dead code warnings in the wasm target.
- The `Error`'s kind is a now a set of variants depending on the context
of when an error could occur.
- If another error was the cause, it is now always the `source`.
Along with the `is_*` methods, this should help in understanding *when*
a certain error occurred. For example, an error setting the TLS
certificates will return a builder error, with the TLS error as the
source. This should help differentiate from a TLS error that happens
when connecting to a server.
It also makes the internal code less dependent on all the exact
dependencies that can be enabled or disabled.
