Sema: rewrite peer type resolution

The existing logic for peer type resolution was quite convoluted and
buggy. This rewrite makes it much more resilient, readable, and
extensible. The algorithm works by first iterating over the types to
select a "strategy", then applying that strategy, possibly applying peer
resolution recursively.

The new semantics around PTR for comptime-known fixed-width integers did
require some small changes to parts of the compiler, std, and
compiler_rt. Regarding the MachO changes, I spoke to @kubkon about how
best to do them, and this is what he suggested.

Several new tests have been added to cover cases which the old logic did
not correctly handle.

Resolves: ziglang#9917
Resolves: ziglang#15644
Resolves: ziglang#15709

lib/compiler_rt/exp2.zig

@@ -88,7 +88,7 @@ pub fn exp2f(x: f32) callconv(.C) f32 {
 }
 
 pub fn exp2(x: f64) callconv(.C) f64 {
-    const tblsiz: u32 = @intCast(u32, exp2dt.len / 2);
+    const tblsiz: u16 = exp2dt.len / 2;
     const redux: f64 = 0x1.8p52 / @intToFloat(f64, tblsiz);
     const P1: f64 = 0x1.62e42fefa39efp-1;
     const P2: f64 = 0x1.ebfbdff82c575p-3;

lib/std/Build/Step/CheckObject.zig

@@ -503,7 +503,7 @@ const MachODumper = struct {
                     }
                     try writer.print(" {s}\n", .{sym_name});
                 } else if (sym.undf()) {
-                    const ordinal = @divTrunc(@bitCast(i16, sym.n_desc), macho.N_SYMBOL_RESOLVER);
+                    const ordinal = sym.getDylibOrdinal();
                     const import_name = blk: {
                         if (ordinal <= 0) {
                             if (ordinal == macho.BIND_SPECIAL_DYLIB_SELF)

lib/std/fmt/parse_float/parse.zig

@@ -156,7 +156,7 @@ fn parsePartialNumberBase(comptime T: type, stream: *FloatStream, negative: bool
         };
     }
 
-    n_digits -= info.max_mantissa_digits;
+    n_digits -= @as(isize, info.max_mantissa_digits);
     var many_digits = false;
     stream.reset(); // re-parse from beginning
     while (stream.firstIs3('0', '.', '_')) {

lib/std/macho.zig

@@ -890,6 +890,10 @@ pub const nlist_64 = extern struct {
         if (!sym.undf()) return false;
         return sym.n_value != 0;
     }
+
+    pub fn getDylibOrdinal(sym: nlist_64) i16 {
+        return @divTrunc(@bitCast(i16, sym.n_desc), @as(u15, N_SYMBOL_RESOLVER));
+    }
 };
 
 /// Format of a relocation entry of a Mach-O file.  Modified from the 4.3BSD