Default to re2 parser is available

After benchmarking, the results are out, at least on the current
sample file:

First, re2 is ridiculously faster than the basic parser, even with
tons of caching. re2 does benefit from caching, but it's so fast that
it needs very high hitrates (so a very large cache) for the caching to
have a real impact, it's fast enough that at low hitrates (small
sizes) the cache does slow down parsing visibly which is not the case
of the basic parser.

Second, LRU is confirmed to be a better cache replacement policy than
clearing (which... duh), it's not super sensible at very low sizes but
at 100 entries it starts really pulling ahead, so definitely the
better default at 200 (where even with the overhead of the more
layered approach it's ahead of the legacy parser and its immutable 20
entries clearing cache).

The locking doesn't seem to have much impact without contention, and
even contended the LRU seems to behave way better than the clearing
cache still. So fallback onto locked LRU if re2 is not available.

src/ua_parser/__init__.py

@@ -46,7 +46,9 @@
 
 VERSION = (1, 0, 0)
 
-from typing import Optional
+import contextlib
+from typing import Callable, Optional, Type
+
 from .core import (
     DefaultedParseResult,
     Device,
@@ -65,17 +67,24 @@
 from .caching import CachingParser, Clearing, LRU, Locking
 from .loaders import load_builtins, load_data, load_yaml
 
+Re2Parser: Optional[Callable[[Matchers], Parser]] = None
+with contextlib.suppress(ImportError):
+    from .re2 import Parser as Re2Parser
+
 
 parser: Parser
 
 
 def __getattr__(name: str) -> Parser:
     global parser
     if name == "parser":
-        parser = CachingParser(
-            BasicParser(load_builtins()),
-            LRU(200),
-        )
+        if Re2Parser is not None:
+            parser = Re2Parser(load_builtins())
+        else:
+            parser = CachingParser(
+                BasicParser(load_builtins()),
+                Locking(LRU(200)),
+            )
         return parser
     raise AttributeError(f"module {__name__!r} has no attribute {name!r}")