Move entry_block() into Layout.

The single entry block in a function is simply the first block in the layout.

Remove the 'entry' keyword from the textual IL, the lexer and parser.

docs/cton_lexer.py

@@ -26,7 +26,7 @@ class CretonneLexer(RegexLexer):
             (r'[-+]?(\d+\.\d+([eE]\d+)?|[sq]NaN|Inf)', Number.Float),
             (r'[-+]?\d+', Number.Integer),
             # Reserved words.
-            (keywords('function', 'entry'), Keyword),
+            (keywords('function'), Keyword),
             # Known attributes.
             (keywords('align', 'aligntrap', 'uext', 'sext', 'inreg'), Name.Attribute),
             # Well known value types.