java.io: full File API + byte/char streams over Chez ports

Expand java.io so libraries that touch the filesystem work unchanged.

File: the full method surface — length, lastModified, can{Read,Write,Execute},
isHidden, list, mkdir(s), delete, createNewFile, renameTo, getParentFile,
get{Absolute,Canonical}File, compareTo/equals/hashCode — plus the statics
separator / pathSeparator / createTempFile / listRoots. A File now keeps the
path as given (new File("rel").getPath() is "rel", .isAbsolute false); a
relative path resolves against JOLT_PWD only when the filesystem is touched,
matching the JVM. slurp/spit and the dir helpers go through the same
resolution, fixing a spit-vs-slurp inconsistency.

Streams (host/chez/io-streams.ss) — each a jhost wrapping a Chez port, so
buffering, EOF and binary<->char transcoding come from Chez:
- FileInputStream / FileOutputStream / ByteArrayInputStream /
  ByteArrayOutputStream / BufferedInputStream / BufferedOutputStream
- FileReader / FileWriter / InputStreamReader / OutputStreamWriter /
  BufferedReader / BufferedWriter
Buffered* return the wrapped stream (Chez ports are already buffered).

clojure.java.io: input-stream/output-stream now yield real byte streams (were
aliased to the char reader/writer); added copy (byte-exact for byte sources),
make-parents, delete-file. with-open also closes file-writer/port-writer/
print-writer (a pre-existing gap).

All runtime shims, no re-mint. 15 JVM-certified corpus rows; make test +
shakesmoke green.

host/chez/rt.ss

@@ -376,6 +376,11 @@
 ;; it. After the dispatchers it chains.
 (load "host/chez/natives-array.ss")
 
+;; java.io byte/char streams (FileInputStream/…/ByteArrayOutputStream/Buffered*)
+;; over Chez ports. After io.ss (extends its slurp/__close/reader-jhost?) and
+;; natives-array.ss (the byte-array <-> bytevector bridge).
+(load "host/chez/io-streams.ss")
+
 ;; clojure.lang.PersistentQueue: a functional queue + EMPTY static.
 ;; Chains seq/count/empty?/peek/pop/conj/sequential?/class/instance?/printer, so
 ;; load after natives-array (the dispatchers it extends).