Port read/write functions take bytevectors

This will allow better Scheme integration for ports.

* libguile/ports.h (scm_t_port_buffer): Change "holder" member to be a
  bytevector defined to have "buf" as its starting point.
  (scm_t_ptob_descriptor): Change read and write functions to take
  bytevectors as arguments and to return the number of octets read or
  written.
  (scm_make_port_type): Adapt accordingly.
  (scm_c_read_bytes, scm_c_write_bytes): New functions that take
  bytevectors.
* libguile/ports.c (scm_make_port_type): Adapt to read/write function
  prototype change.
  (scm_c_make_port_buffer): Arrange to populate the "bytevector" field.
  (scm_i_read_bytes_unlocked): New function.
  (scm_i_read_unlocked): Use scm_i_read_bytes_unlocked.
  (scm_c_read_bytes_unlocked): New function.
  (scm_c_read_unlocked): Update comment, and always go through the
  buffer.
  (scm_c_read_bytes): New function.
  (scm_flush_unlocked): Use scm_i_write_unlocked instead of the port's
  write function.
  (scm_i_write_bytes_unlocked): New function.
  (scm_i_write_unlocked): Use scm_i_write_bytes_unlocked.
  (scm_c_write_bytes_unlocked): New function.
  (scm_c_write_unlocked): Always write through the buffer.
  (scm_c_write_bytes): New function.
  (scm_truncate_file): Remove unused variable.
  (void_port_read, void_port_write): Adapt to read/write prototype
  change.
* libguile/fports.c (fport_read, fport_write):
* libguile/r6rs-ports.c (bytevector_input_port_read)
  (custom_binary_input_port_read, bytevector_output_port_write)
  (custom_binary_output_port_write, transcoded_port_write)
  (transcoded_port_read): Adapt to read/write prototype
  change.
  (scm_get_bytevector_n, scm_get_bytevector_n_x)
  (scm_get_bytevector_all): Use scm_c_read_bytes.
  (scm_put_bytevector): Use scm_c_write_bytes.
* libguile/strports.c (string_port_read, string_port_write):
* libguile/vports.c (soft_port_write, soft_port_read): Adapt to
  read/write prototype change.
* test-suite/standalone/test-scm-c-read.c (custom_port_read): Fix for
  read API change.

libguile/ports.h

@@ -87,8 +87,8 @@ typedef struct
      peek-u8 should still return EOF.  */
   int has_eof;
 
-  /* Heap object that keeps `buf' alive. */
-  void *holder;
+  /* Bytevector whose contents are [BUF, BUF + SIZE). */
+  SCM bytevector;
 } scm_t_port_buffer;
 
 
@@ -204,8 +204,8 @@ typedef struct scm_t_ptob_descriptor
   char *name;
   int (*print) (SCM exp, SCM port, scm_print_state *pstate);
 
-  void (*read) (SCM port, scm_t_port_buffer *dst);
-  void (*write) (SCM port, scm_t_port_buffer *src);
+  size_t (*read) (SCM port, SCM dst, size_t start, size_t count);
+  size_t (*write) (SCM port, SCM src, size_t start, size_t count);
   scm_t_off (*seek) (SCM port, scm_t_off OFFSET, int WHENCE);
   void (*close) (SCM port);
 
@@ -230,8 +230,8 @@ SCM_API scm_t_ptob_descriptor* scm_c_port_type_ref (long ptobnum);
 SCM_API long scm_c_port_type_add_x (scm_t_ptob_descriptor *desc);
 SCM_API scm_t_bits scm_make_port_type
 	(char *name,
-         void (*read) (SCM port, scm_t_port_buffer *dst),
-         void (*write) (SCM port, scm_t_port_buffer *src));
+         size_t (*read) (SCM port, SCM dst, size_t start, size_t count),
+         size_t (*write) (SCM port, SCM src, size_t start, size_t count));
 SCM_API void scm_set_port_print (scm_t_bits tc,
 				 int (*print) (SCM exp,
 					       SCM port,
@@ -323,6 +323,7 @@ SCM_INLINE int scm_peek_byte_or_eof_unlocked (SCM port);
 SCM_API int scm_slow_peek_byte_or_eof_unlocked (SCM port);
 SCM_API size_t scm_c_read (SCM port, void *buffer, size_t size);
 SCM_API size_t scm_c_read_unlocked (SCM port, void *buffer, size_t size);
+SCM_API size_t scm_c_read_bytes (SCM port, SCM dst, size_t start, size_t count);
 SCM_API scm_t_wchar scm_getc (SCM port);
 SCM_API scm_t_wchar scm_getc_unlocked (SCM port);
 SCM_API SCM scm_read_char (SCM port);
@@ -359,6 +360,7 @@ SCM_API void scm_puts (const char *str_data, SCM port);
 SCM_INLINE void scm_puts_unlocked (const char *str_data, SCM port);
 SCM_API void scm_c_write (SCM port, const void *buffer, size_t size);
 SCM_API void scm_c_write_unlocked (SCM port, const void *buffer, size_t size);
+SCM_API void scm_c_write_bytes (SCM port, SCM src, size_t start, size_t count);
 SCM_API void scm_lfwrite (const char *ptr, size_t size, SCM port);
 SCM_API void scm_lfwrite_unlocked (const char *ptr, size_t size, SCM port);
 SCM_INTERNAL void scm_lfwrite_substr (SCM str, size_t start, size_t end,