Bug 7558: Complete implementation of the boot-time symbol table

* sources/dfmc/llvm-back-end/llvm-primitives-symbol.dylan
  (op--lookup-symbol): Complete implementation of linear search of %oblist.
  (op--as-lowercase): New auxiliary emitter method.
  (primitive primitive-strncasecmp): New auxiliary primitive for
   case-insensitive characater comparisons.
  (op--register-symbol): Complete implementation of adding symbols to %oblist.
  (primitive primitive-resolve-symbol): Complete emitter implementation.
  (primitive primitive-string-as-symbol): Complete emitter implementation.
  (primitive primitive-preboot-symbols): Complete emitter implementation.

* sources/app/llvm-runtime-generator/llvm-runtime-generator.dylan
  ($wrapper-classes): Add <symbol> to the list of wrappers referenced
   by the runtime.
Bug 7458: Add a class wrapper retrieval emitter method

* sources/dfmc/llvm-back-end/llvm-ops.dylan
  (op--object-mm-wrapper): New method for emitting code to retrieve the
   <mm-wrapper> for an object, applicable to both heap objects (where it
   is stored in the zeroth slot) and to tagged direct objects
   (where the wrappers can be retrieved by indexing into
    $direct-object-mm-wrappers).

* sources/app/llvm-runtime-generator/llvm-runtime-generator.dylan
  ($runtime-referenced-variables): New constant representing
   variables (such as $direct-object-mm-wrappers) in the Dylan library
   referenced by the generated run-time.
  (generate-runtime-heap): Add code to emit extern declarations for variables
   referenced from the generated run-time.
Bug 7458: Extract a method for subtype bit based instance checks

* sources/dfmc/llvm-back-end/llvm-emit-type-check.dylan
  (do-emit-instance-cmp on <&class>): Split out instance checks for heap
   objects based on mm-wrapper-subtype-mask.
  (op--heap-object-subtype-bit-instance-cmp): New method.
Bug 7458: Fix return value of setter primitives

* sources/dfmc/llvm-back-end/llvm-primitives-basic.dylan
  (primitive primitive-vector-element-setter): Return new-value.
  (primitive primitive-slot-value-setter): Return value.
  (primitive primitive-repeated-slot-value-setter): Return value.
Bug 7458: Snapshot