branch API
branch tests
use fold API for references

This reworks the references API to provide `reference-fold` and define
`references` over that, changes the refs.h bindings to look more like
those of branch.h, and renames the `ref-type` type to `reference-type`
in lolevel.
update branch & reference docs, other minor doc fixes
update reference tests to use correct type symbols
Merge branch 'development'
doc branch-name, fix docs for branch-rename & branch-delete
remove debug args from csc invocations in setup file
avoid redefinition of repository-head (use lolevel method directly)
don't rely on tag order in tests
release updates for reference & branch APIs
config-set -> config-set! (and unset -> unset!) in documentation