File: precompute hash

Most files are going to be looked up into a set or dictionary.  Precompute
the hash so that we only need to do so once and we can also use it to
quickly weed out unequal objects.

On a QEMU build, the time spent in __eq__ and __hash goes respectively
from 3.110s to 2.162s and from 0.648s to 0.299s.  Even larger gains are
obtained by the next patch.

mesonbuild/mesonlib.py

@@ -242,6 +242,7 @@ class File:
         self.is_built = is_built
         self.subdir = subdir
         self.fname = fname
+        self.hash = hash((is_built, subdir, fname))
 
     def __str__(self) -> str:
         return self.relative_name()
@@ -291,10 +292,12 @@ class File:
     def __eq__(self, other) -> bool:
         if not isinstance(other, File):
             return NotImplemented
+        if self.hash != other.hash:
+            return False
         return (self.fname, self.subdir, self.is_built) == (other.fname, other.subdir, other.is_built)
 
     def __hash__(self) -> int:
-        return hash((self.fname, self.subdir, self.is_built))
+        return self.hash
 
     @lru_cache(maxsize=None)
     def relative_name(self) -> str: