SBCL 2.6.6

• minor incompatible change: FDEFINITION now returns the outermost wrapper
  (added e.g. by TRACE, PROFILE) like SYMBOL-FUNCTION. (lp#799533)
• minor incompatible change: in unsafe code, C strings with :EXTERNAL-FORMAT
  :ASCII are copied directly as byte-sized quantities without checking
  whether the top bit of the byte is set.
• platform support:
  ** fix the build on big-endian 64-bit PowerPC with ELFv2. (thanks to
  Piotr Kubaj)
  ** move the static space address for macOS 27 on ARM64. (lp#2156072,
  reported by Gary Palter)
  ** optimizations to SB-THREAD:BARRIER for ARM64. (thanks to Sahil Kang)
  ** fix a compiler crash in MULTIPLE-VALUE-LIST in argument forms on ARM64.
  (lp#2155788, reported by Gary Palter)
• bug fix: TRACE no longer fails when trying to print a return value that
  cannot be printed readably and PRINT-READABLY is true.
• optimization: the compiler is more precise in its type derivation of
  COERCE given constraints on its inputs.
• optimization: the compiler is better able to derive the return types of
  AREF and ELT.
• optimization: faster encoding and decoding of UTF-8 C strings.
• optimization: (length (intersection a b)) doesn't cons an intermediate
  list.
• documentation: the manual now includes a section for SB-INTROSPECT, which
  has also seen improvement in its documentation strings and comments.
• documentation: fixed many typesetting problems and typos in the user
  manual.