py: Start implementing "struct" module.

Only calcsize() and unpack() functions provided so far, for little-endian
byte order. Format strings don't support repition spec (like "2b3i").

Unfortunately, dealing with all the various binary type sizes and alignments
will lead to quite a bloated "binary" helper functions  - if optimizing for
speed. Need to think if using dynamic parametrized algos makes more sense.

py/modstruct.c

@@ -0,0 +1,94 @@
+#include <assert.h>
+#include <string.h>
+#include "misc.h"
+#include "mpconfig.h"
+#include "qstr.h"
+#include "obj.h"
+#include "builtin.h"
+#include "objtuple.h"
+#include "binary.h"
+
+#if MICROPY_ENABLE_MOD_STRUCT
+
+STATIC char get_fmt_type(const char **fmt) {
+    char t = **fmt;
+    switch (t) {
+        case '!':
+            t = '>';
+            break;
+        case '@':
+        case '=':
+        case '<':
+        case '>':
+            break;
+        default:
+            return '@';
+    }
+    // Skip type char
+    (*fmt)++;
+    return t;
+}
+
+STATIC uint calcsize_items(const char *fmt) {
+    // TODO
+    return strlen(fmt);
+}
+
+STATIC mp_obj_t struct_calcsize(mp_obj_t fmt_in) {
+    const char *fmt = mp_obj_str_get_str(fmt_in);
+    char fmt_type = get_fmt_type(&fmt);
+    assert(fmt_type == '<'); (void)fmt_type;
+    uint size;
+    for (size = 0; *fmt; fmt++) {
+        int sz = mp_binary_get_size(*fmt);
+        // TODO
+        assert(sz != -1);
+        size += sz;
+    }
+    return MP_OBJ_NEW_SMALL_INT(size);
+}
+MP_DEFINE_CONST_FUN_OBJ_1(struct_calcsize_obj, struct_calcsize);
+
+STATIC mp_obj_t struct_unpack(mp_obj_t fmt_in, mp_obj_t data_in) {
+    // TODO: "The buffer must contain exactly the amount of data required by the format (len(bytes) must equal calcsize(fmt))."
+    const char *fmt = mp_obj_str_get_str(fmt_in);
+    char fmt_type = get_fmt_type(&fmt);
+    assert(fmt_type == '<'); (void)fmt_type;
+    uint size = calcsize_items(fmt);
+    mp_obj_tuple_t *res = mp_obj_new_tuple(size, NULL);
+    buffer_info_t bufinfo;
+    mp_get_buffer_raise(data_in, &bufinfo);
+    byte *p = bufinfo.buf;
+
+    for (uint i = 0; i < size; i++) {
+        mp_obj_t item = mp_binary_get_val_unaligned_le(*fmt++, &p);
+        res->items[i] = item;
+    }
+    return res;
+}
+MP_DEFINE_CONST_FUN_OBJ_2(struct_unpack_obj, struct_unpack);
+
+STATIC const mp_map_elem_t mp_module_struct_globals_table[] = {
+    { MP_OBJ_NEW_QSTR(MP_QSTR___name__), MP_OBJ_NEW_QSTR(MP_QSTR_struct) },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_calcsize), (mp_obj_t)&struct_calcsize_obj },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_unpack), (mp_obj_t)&struct_unpack_obj },
+};
+
+STATIC const mp_obj_dict_t mp_module_struct_globals = {
+    .base = {&mp_type_dict},
+    .map = {
+        .all_keys_are_qstrs = 1,
+        .table_is_fixed_array = 1,
+        .used = sizeof(mp_module_struct_globals_table) / sizeof(mp_map_elem_t),
+        .alloc = sizeof(mp_module_struct_globals_table) / sizeof(mp_map_elem_t),
+        .table = (mp_map_elem_t*)mp_module_struct_globals_table,
+    },
+};
+
+const mp_obj_module_t mp_module_struct = {
+    .base = { &mp_type_module },
+    .name = MP_QSTR_struct,
+    .globals = (mp_obj_dict_t*)&mp_module_struct_globals,
+};
+
+#endif