Assignment expression code optimization x 1.

This commit is the first in a commit chain significantly optimizing
generation of **assignment expressions** (i.e., PEP 572-compliant Python
expressions containing the ":=" walrus operator) throughout code
snippets dynamically generated by @beartype's type-checking code
generator. Since strings are expensive to repeatedly generate, @beartype
now internally caches and reuses frequently used local variables across
assignment expressions. In truth, this means nothing. But @leycec just
*had* to do this to satisfy his aberrant OCD disorder. I swear! It isn't
my fault! It's fast or it ain't @beartype. (*Tsundera or tundra!?*)

beartype/_check/checkmagic.py

@@ -148,15 +148,15 @@
 '''
 
 # ....................{ NAMES ~ var : pith                 }....................
-VAR_NAME_PREFIX_PITH = f'{NAME_PREFIX}pith_'
+VAR_NAME_PITH_PREFIX = f'{NAME_PREFIX}pith_'
 '''
 Substring prefixing all local variables providing a **pith** (i.e., either the
 current parameter or return value *or* item contained in the current parameter
 or return value being type-checked by the current call).
 '''
 
 
-VAR_NAME_PITH_ROOT = f'{VAR_NAME_PREFIX_PITH}0'
+VAR_NAME_PITH_ROOT = f'{VAR_NAME_PITH_PREFIX}0'
 '''
 Name of the local variable providing the **root pith** (i.e., value of the
 current parameter or return value being type-checked by the current call).