Some slightly performance & style updates

[yhx-12243]

• In the function fn as_bytes in impl CompactString, there is an redundant slicing (with a self.len() call) which is unnecessary after the update c95edd5.
• This crate has documented that for 64-bit architectures the capacity will be always inlined. So the logic of is_heap can be replaced with directly returning false to reduce some code generation.