[goalc] Cleaned up speedups (#3066)

Started at 349,880,038 allocations and 42s

- Switched to making `Symbol` in GOOS be a "fixed type", just a wrapper
around a `const char*` pointing to the string in the symbol table. This
is a step toward making a lot of things better, but by itself not a huge
improvement. Some things may be worse due to more temp `std::string`
allocations, but one day all these can be removed. On linux it saved
allocations (347,685,429), and saved a second or two (41 s).
- cache `#t` and `#f` in interpreter, better lookup for special
forms/builtins (hashtable of pointers instead of strings, vector for the
small special form list). Dropped time to 38s.
- special-case in quasiquote when splicing is the last thing in a list.
Allocation dropped to 340,603,082
- custom hash table for environment lookups (lexical vars). Dropped to
36s and 314,637,194
- less allocation in `read_list` 311,613,616. Time about the same.
- `let` and `let*` in Interpreter.cpp 191,988,083, time down to 28s.

test/test_reader.cpp

@@ -26,7 +26,7 @@ bool check_first_float(Object o, double x) {
 }
 
 bool check_first_symbol(Object o, const std::string& sym) {
-  return o.as_pair()->cdr.as_pair()->car.as_symbol()->name == sym;
+  return o.as_pair()->cdr.as_pair()->car.as_symbol() == sym;
 }
 
 bool check_first_string(Object o, const std::string& str) {