Add ihex_rs_from_mem() for unterminated string data.

Determine string length of given data in ihex_rs_from_string() and use
new ihex_rs_from_mem() for actual implementation.  Pass file size to
ihex_rs_from_mem() in ihex_rs_from_file(), as files are rarely
NUL-terminated.
Avoid duplicate calculation of record length.

Use record length value already passed as parameter to
ihex_parse_single_record() instead of calculating it again.
Add proper end-of-data checks during parsing.

The size parameter given to ihex_rs_from_mem() needs to be checked to
avoid access beyond the valid data range.  The old checks for \0 bytes
are also left intact.
Add ihex_rs_from_mem() as public API function.
Remove trailing newlines from error strings in ihex_copy.c.

The error messages are likely to be used within another message
string, where the newlines can be very disturbing.  This also unifies
error messages between ihex_copy.c and other sources.
Remove trailing punctuation from error messages.

Being a library, the generated error messages will likely be used
within another message context (such as parentheses).  This error
message convention is in compliance with the C library's strerror()
function.
Introduce IHEX_ORDER_NATIVE.

Specifying IHEX_ORDER_NATIVE with ihex_mem_copy() currently behaves
just as IHEX_ORDER_LITTLEENDIAN.  This is a portability issue and
needs to be addressed separately.
Add ihex_rs_free() implementation.

Code shamelessly copied from mock_recordset_free() with a few
additional safety checks.
Add ihex_rs_free() to the public library API.
Use new ihex_rs_free() implementation in tests.
Merge pull request #10 from acolomb/fix-unterminated-parsing

Fix parsing of unterminated data in memory
Merge pull request #11 from acolomb/errmsg-formatting

Error message formatting
Add rudimentary unit test for ihex_rs_free.
Merge pull request #15 from acolomb/recordset-free

Implement ihex_rs_free()

Conflicts:
	tests/test_copy.c
Merge pull request #16 from acolomb/byteorder-unchanged

Introduce IHEX_ORDER_NATIVE.