The three binary-packing functions from Lua are now implemented in
a dedicated `string_pack.ts` module and wired into `stringApi`.
Format parsing follows the exact option set of Lua:
* signed and unsigned integers of configurable width
(`b`/`B`/`h`/`H`/`l`/`L`/`j`/`J`/`T`/`i`/`I`),
* IEEE 754 single and double precision floats (`f`/`d`/`n`),
* fixed-length strings (`c`),
* length-prefixed strings (`s`),
* zero-terminated strings (`z`),
* padding bytes (`x`),
* alignment padding (`X`), and
* endianness/alignment modifiers (`<`/`>`/`=`/`!`).
`string.packsize` rejects variable-length formats (`s` and `z`) as Lua
requires.
`string.unpack` returns all unpacked values followed by the
next position integer, matching the Lua contract exactly.
Test suite extended in separates `string_pack_test.lua` file. Passes
both Lua and Space Lua execution.
Signed-off-by: Matouš Jan Fialka <mjf@mjf.cz>
