v0.10.2: parser drop

v0.9.0: vm tail, contextvars, and the runtime time layer

This is the release that closes out the Tier-1 dispatch panel. After
v0.9 every bytecode the compiler routinely emits has a real arm in
the eval loop. Generators run on their own goroutine and synchronise
with the caller through buffered channels. RETURN_GENERATOR detaches
the current frame from the chunk arena and spawns the goroutine;
SEND, YIELD_VALUE, and CLEANUP_THROW route through that channel pair.
Pattern matching covers all four MATCH_* opcodes, including the
__match_args__ extraction path. Set builders, IMPORT_STAR, and
WITH_EXCEPT_START are also live.

Three packages land underneath the vm work. hamt is the immutable
hash-array mapped trie that backs contextvars.Context. contextvar
implements PEP 567 set/reset semantics with copy-on-write. pytime
gives the runtime a real nanosecond clock layer (with rounding modes
matching CPython byte-for-byte) and that clock now powers the GIL
switch interval through a per-thread gilSwitchTimer.

Two more pieces of infrastructure close out the small-runtime shelf:
getopt replaces the ad-hoc CLI switch in cmd/gopy with a real
POSIX-style argv walker, and hashtable provides the generic hash
table that runtime code needs outside the dict object. tokenize.Iter
graduates from a hand-rolled splitter to the real lexer state
machine.

Known gaps for v0.10:
- Sub-interpreters and Py_NewInterpreter. The GIL-drop wiring is in;
  the contention path activates with sub-interpreters.
- Full async iterator / awaitable surface (GET_AITER, GET_ANEXT,
  END_ASYNC_FOR). v0.9 ships error-returning stubs.
- __match_args__ MRO walk. v0.9 walks type identity plus one level
  of Bases; full MRO arrives with the type-system port.
- WITH_EXCEPT_START traceback argument (becomes real when the
  traceback object lands).
- SEND_GEN specialisation.
- Real frozen importlib code objects in imp/frozen_bootstrap.go.

See changelog/v0.9.0.md for the full file-by-file breakdown.

v0.8.1: CI fixes (lint + Windows build)

v0.8.0 - the import drop

marshal can round-trip .pyc files (PEP 552 timestamp and hash
variants), the full TYPE_LONG/TYPE_CODE/TYPE_SET/TYPE_DICT/
TYPE_BINARY_COMPLEX type tags, and FLAG_REF back-references.

The codecs package has a registry and built-in utf-8, ascii, and
latin-1 codecs with alias normalization.

The imp package implements the full import path: frozen module table,
sys.modules cache, inittab, ExecCodeModule, source and .pyc loaders,
and ImportModuleLevel with the sys.modules -> frozen -> built-in
lookup chain. Relative imports resolve via resolveAbsName.

IMPORT_NAME and IMPORT_FROM bytecodes in the eval loop now call into
imp through the Executor interface, so `import foo` and
`from foo import bar` work end-to-end for built-in and frozen modules.