sys.path is not initialized by rt_init(), that's left for platform-specific
startup code. (For example, bare metal port may have some hardcoded defaults,
and let user change sys.path directly; while port for OS with environment
feature can take path from environment). If it's not explicitly initialized,
modules will be imported only from a current directory.
In Python, importing module several times returns same underlying module
object. This also fixes import statement handling for builtin modules.
There're still issues:
1. CPython exposes set of loaded modules as sys.modules, we may want to
do that either.
2. Builtin modules are implicitly imported, which is not really correct.
We should separate registering a (builtin) module and importing a module.
CPython keeps builtin module names in sys.builtin_module_names .
Qstr's are now split into a linked-list of qstr pools. This has 2
benefits: the first pool can be in ROM (huge benefit, since we no longer
use RAM for the core qstrs), and subsequent pools use m_new for the next
pool instead of m_renew (thus avoiding a huge single table for all the
qstrs).
Still would be better to use a hash table, but this scheme takes us part
of the way (eventually convert the pools to hash tables).
Also fixed bug with import.
Also improved the way the module code is referenced (not magic number 1
anymore).
