refactor: project configuration get/set parity

[scolsen]

This commit refactors our handling of project configuration
getters/setters in hopes of making it easier to ensure fields that have
getters have setters and vice versa, and to ensure field keys are
consistent across the Project.config and get-config commands.

This commit also makes these commands return XObjs for all calls,
instead of coercing certain things into strings and also adds getters
for fields that previously lacked them, e.g.:

(Project.get-config "libflags")
=> ("-lm")

I've maintained parity with existing behaviors to avoid breaking
changes, thus, this works as it did before this commit:

(Project.config "libflags" "-lfoo")
(Project.get-config "libflags")
=> ("-lm" "-lfoo")

though it could now arguably take a list as an argument to permit
complete one-liner replacements; one could still add a single flag and
the interface would be more functional:

   (Project.config "libflags"
     (cons "-lfoo" (Project.get-config "libflags")))
   => ("-lm" "-lfoo")
   (Project.config "libflags" ("-lbar" "-qux"))
   => ("-lbar" "-lqux")

But, again, I've retained the old behavior to avoid breaking existing
builds.

This also fixes a small issue where by setting the file path print
length to "full" still resulted in the "short" printing scheme.

[scolsen]

I should also note that we now better account for a notion of "read-only" fields:

鲤 (Project.config "load-stack" "foo")
[CONFIG ERROR] the configuration field load-stack is read-only

And not all fields have to have getters/setters either, some have neither -- again, I tried to keep things matching the current behavior, but we might want to make more fields accessible through "read-only" getters now (e.g. Project.files):

鲤 (Project.get-config "files")
[CONFIG ERROR] Project.get-config can't understand the key 'files at REPL:3:21.

Traceback:
  (Project.get-config "files") at REPL:3:1.

[eriksvedang]

Nice improvements!

[eriksvedang]

Awesome!