Optimization passes

[rdunnington]

• New zig build asm command to generate assembly for perf analysis
• Gate the debug trace and trap features behind a compile-time option. Most users probably don't care about this and it has a non-trivial impact on perf.
• Fix instruction immediates regressing to 32 bytes. When they were changed to be extern structs, the ordering started to matter. Now there's a comptime check to make sure they're always 16 bytes.
• Embed param/return count in function instance to avoid an extra pointer hop.
• Minor optimization to use @memset to initialize locals since the default values are always 0 anyway.