When opts.map is undefined and the root has no previous source map,
skip MapGenerator construction entirely and stringify directly. This
avoids 3 Map allocations, a clearAnnotation AST walk, and a previous()
AST walk per file.

Cache node.raws references locally in hot methods (atrule, block, body,
comment, decl) to avoid repeated property chain resolution. In decl(),
inline the common rawValue() path when node.raws.value exists. In raw(),
cache root.rawCache in a local variable. In body(), short-circuit
raw(child, 'before') when child.raws.before is already defined. In
block() and comment(), inline the raws check to skip raw() call overhead
when the value is already present.