Refactor configuration file parsing

Now the time for importing the kitty.config module has been halved, from
16ms from 32ms on my machine. Also, the new architecture will eventually
allow for auto generating a bunch of python-to-C boilerplate code.

kittens/diff/options/utils.py

@@ -0,0 +1,62 @@
+#!/usr/bin/env python
+# vim:fileencoding=utf-8
+# License: GPLv3 Copyright: 2021, Kovid Goyal <kovid at kovidgoyal.net>
+
+
+from typing import Any, Dict, Iterable, Sequence, Tuple, Union
+
+from kitty.conf.utils import KittensKeyDefinition, key_func, parse_kittens_key
+
+func_with_args, args_funcs = key_func()
+FuncArgsType = Tuple[str, Sequence[Any]]
+
+
+@func_with_args('scroll_by')
+def parse_scroll_by(func: str, rest: str) -> Tuple[str, int]:
+    try:
+        return func, int(rest)
+    except Exception:
+        return func, 1
+
+
+@func_with_args('scroll_to')
+def parse_scroll_to(func: str, rest: str) -> Tuple[str, str]:
+    rest = rest.lower()
+    if rest not in {'start', 'end', 'next-change', 'prev-change', 'next-page', 'prev-page', 'next-match', 'prev-match'}:
+        rest = 'start'
+    return func, rest
+
+
+@func_with_args('change_context')
+def parse_change_context(func: str, rest: str) -> Tuple[str, Union[int, str]]:
+    rest = rest.lower()
+    if rest in {'all', 'default'}:
+        return func, rest
+    try:
+        amount = int(rest)
+    except Exception:
+        amount = 5
+    return func, amount
+
+
+@func_with_args('start_search')
+def parse_start_search(func: str, rest: str) -> Tuple[str, Tuple[bool, bool]]:
+    rest_ = rest.lower().split()
+    is_regex = bool(rest_ and rest_[0] == 'regex')
+    is_backward = bool(len(rest_) > 1 and rest_[1] == 'backward')
+    return func, (is_regex, is_backward)
+
+
+def syntax_aliases(raw: str) -> Dict[str, str]:
+    ans = {}
+    for x in raw.split():
+        a, b = x.partition(':')[::2]
+        if a and b:
+            ans[a.lower()] = b
+    return ans
+
+
+def parse_map(val: str) -> Iterable[KittensKeyDefinition]:
+    x = parse_kittens_key(val, args_funcs)
+    if x is not None:
+        yield x