all: AVX-512 (#217)

Extends avo to support most AVX-512 instruction sets.

The instruction type is extended to support suffixes. The K family of opmask
registers is added to the register package, and the operand package is updated
to support the new operand types. Move instruction deduction in `Load` and
`Store` is extended to support KMOV* and VMOV* forms.

Internal code generation packages were overhauled. Instruction database loading
required various messy changes to account for the additional complexities of the
AVX-512 instruction sets. The internal/api package was added to introduce a
separation between instruction forms in the database, and the functions avo
provides to create them. This was required since with instruction suffixes there
is no longer a one-to-one mapping between instruction constructors and opcodes.

AVX-512 bloated generated source code size substantially, initially increasing
compilation and CI test times to an unacceptable level. Two changes were made to
address this:

1.  Instruction constructors in the `x86` package moved to an optab-based
    approach. This compiles substantially faster than the verbose code
    generation we had before.

2.  The most verbose code-generated tests are moved under build tags and
    limited to a stress test mode. Stress test builds are run on
    schedule but not in regular CI.

An example of AVX-512 accelerated 16-lane MD5 is provided to demonstrate and
test the new functionality.

Updates #20 #163 #229

Co-authored-by: Vaughn Iverson <vsivsi@yahoo.com>

tests/alloc/zeroing/zeroing_test.go

@@ -0,0 +1,29 @@
+package zeroing
+
+import (
+	"testing"
+
+	"golang.org/x/sys/cpu"
+)
+
+//go:generate go run asm.go -out zeroing.s -stubs stub.go
+
+func TestZeroing(t *testing.T) {
+	const (
+		n      = 32
+		expect = n * (n + 1) / 2
+	)
+
+	if !cpu.X86.HasAVX512F {
+		t.Skip("require AVX512F")
+	}
+
+	var got [8]uint64
+	Zeroing(&got)
+
+	for i := 0; i < 8; i++ {
+		if got[i] != expect {
+			t.Errorf("got[%d] = %d; expect %d", i, got[i], expect)
+		}
+	}
+}