ddlog.h: Functions to pass non-C strings to DDlog.

[ryzhyk]

ddlog.h can be used when invoking ddlog from other programming
languages. Such languages - such as Go - may not be able to easily /
cheaply generate a null-terminated string from their own string type.

For example Go provides _GoStringPtr (https://golang.org/cmd/cgo/)
which gives access to the underlying byte array (should be treated as
immutable), but that array may not be null-terminated. Which means that
in order to call ddlog_string, one has to make a copy of the string
first (e.g. with CString) and ensure it is null terminated. Given that
ddlog makes its own copy of the string when building the record, we are
up to 2 copies.

This commit adds three functions that accept non-null-terminated
strings:

/*
 * Create a string value.
 *
 * `s` - points to the start of a UTF8 string.  The string does not have to be
 *       NULL-terminated.
 * `length` - length of string in bytes.
 *
 * This function copies `s` to an internal buffer, so the caller is responsible for
 * deallocating `s` if it was dynamically allocated.
 *
 * Returns `NULL` if `s` is not a valid UTF8 string.
 */
extern ddlog_record* ddlog_string_with_length(const char * s, size_t len);

/*
 * Same as `ddlog_struct()`, but passes constructor name as
 * non-null-terminated string represented by its start address and length in
 * bytes.
 */
extern ddlog_record* ddlog_struct_with_length(const char* constructor,
                                              size_t constructor_len,
                                              ddlog_record ** args,
                                              size_t len);

/*
 * Same as ddlog_struct_static_cons(), but passes constructor name as
 * non-null-terminated string represented by its start address and length in
 * bytes.
 */
extern ddlog_record* ddlog_struct_static_cons_with_length(
        const char *constructor,
        size_t constructor_len,
        ddlog_record **args, size_t args_len);

In addition, ddlog_get_str_with_length() was modified to return string length
along with pointer, which may be more convenient than calling
ddlog_get_strlen() in some cases.

Resolves #515.

[antoninbas]

LGTM, thanks for making this change