Support functions for navigating buffer caches

I've wanted an MRU/LRU sort of my "source code buffers" in Emacs. This commit
support three ways for working with a cache of source code buffers.

So first, what's a source code buffer? Well it isn't a buffer like *Messages*;
we can call these "Emacs-generated" buffers for convenience. Other problematic
buffers are buffers like `magit-status` and `dired-mode` and `erc` buffers.

I added some predicates for querying buffers for their major modes.

Supporting three KBDs for quickly accessing these functions:
1. <SPC><SPC> Toggle previous buffer
2. <SPC>b     Use ivy to fuzzily search source code buffers
3. C-{f,b}    Cycle {forwards,backwards} through the source code buffer cache.

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