Speeding up jenv initialization when using fish

[0az]

Right now, new interactive shells take approximately ~600ms to start in fish. Profiling shows that jenv's init process accounts for approximately 75% (~450ms) of that time, which is further broken down into

• jenv init - itself: ~150ms
• Completions: ~75ms
• jenv commands: ~75 ms
• jenv rehash: ~100ms
• jenv refresh-plugins: ~125ms

There's two optimizations that might help:

1. Replacing source ($command | psub) with $command | source. This avoids the psub temporary file overhead, and is possibly statistically significant outside of microbenchmarks.
2. Lazily calling jenv rehash and jenv refresh-plugins. Do these really need to run on every interactive shell invocation? Is restricting them to login shells only fine? The former's probably fine to drop, as I don't expect that the average user will need rehashes more than once per month or so, which is supported by adoptopenjdk Homebrew Cask's commit history. This isn't perfect, but it's a good estimate for how often people will notice: not very often.