Janet 1.27.0

• Change semantics around bracket tuples to no longer be equal to regular tuples.
• Add index argument to ffi/write for symmetry with ffi/read.
• Add buffer/push-at
• Add ffi/pointer-buffer to convert pointers to buffers the cannot be reallocated. This
  allows easier manipulation of FFI memory, memory mapped files, and buffer memory shared between threads.
• Calling ev/cancel on a fiber waiting on ev/gather will correctly
  cancel the child fibers.
• Add (sandbox ...) function to core for permission based security. Also add janet_sandbox to C API.
  The sandbox allows limiting access to the file system, network, ffi, and OS resources at runtime.
• Add (.locals) function to debugger to see currently bound local symbols.
• Track symbol -> slot mapping so debugger can get symbolic information. This exposes local bindings
  in debug/stack and disasm.
• Add os/compiler to detect what host compiler was used to compile the interpreter
• Add support for mingw and cygwin builds (mingw support also added in jpm).