stage2: first pass at printing AIR/Liveness to text

* some instructions are not implemented yet
 * fix off-by-1 in Air.getMainBody
 * Compilation: use `@import("builtin")` rather than `std.builtin`
   for the values that are different for different build configurations.
 * Sema: avoid calling `addType` in between
   air_instructions.ensureUnusedCapacity and corresponding
   appendAssumeCapacity because it can possibly add an instruction.
 * Value: functions print their names

src/Module.zig

@@ -3551,7 +3551,8 @@ pub fn analyzeFnBody(mod: *Module, decl: *Decl, func: *Fn) SemaError!Air {
     try sema.analyzeFnBody(&inner_block, func.zir_body_inst);
 
     // Copy the block into place and mark that as the main block.
-    try sema.air_extra.ensureUnusedCapacity(gpa, inner_block.instructions.items.len + 1);
+    try sema.air_extra.ensureUnusedCapacity(gpa, @typeInfo(Air.Block).Struct.fields.len +
+        inner_block.instructions.items.len);
     const main_block_index = sema.addExtraAssumeCapacity(Air.Block{
         .body_len = @intCast(u32, inner_block.instructions.items.len),
     });