Indentation code generation optimization.

This commit significantly optimizes usage of indentation (i.e., semantic
whitespace) 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 indentation. 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. (*Ophthalmology or optimality!?*)

beartype/_check/code/codemagic.py

@@ -141,12 +141,13 @@
 '''
 
 
-HINT_META_INDEX_INDENT = next(__hint_meta_index_counter)
+HINT_META_INDEX_INDENT_LEVEL = next(__hint_meta_index_counter)
 '''
-0-based index into each tuple of hint metadata providing **current
-indentation** (i.e., Python code snippet expanding to the current level of
-indentation appropriate for the currently visited hint).
+0-based index into each tuple of hint metadata providing the **current
+indentation level** (i.e., 1-based positive integer describing the current level
+of indentation appropriate for the currently visited hint).
 '''
 
+
 # Delete the above counter for safety and sanity in equal measure.
 del __hint_meta_index_counter