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>

printer/goasm.go

@@ -132,17 +132,18 @@ func (p *goasm) flush() {
 	// considered in this calculation.
 	width := 0
 	for _, i := range p.instructions {
-		if len(i.Operands) > 0 && len(i.Opcode) > width {
-			width = len(i.Opcode)
+		opcode := i.OpcodeWithSuffixes()
+		if len(i.Operands) > 0 && len(opcode) > width {
+			width = len(opcode)
 		}
 	}
 
 	// Output instruction block.
 	for _, i := range p.instructions {
 		if len(i.Operands) > 0 {
-			p.Printf("\t%-*s%s\n", width+1, i.Opcode, joinOperands(i.Operands))
+			p.Printf("\t%-*s%s\n", width+1, i.OpcodeWithSuffixes(), joinOperands(i.Operands))
 		} else {
-			p.Printf("\t%s\n", i.Opcode)
+			p.Printf("\t%s\n", i.OpcodeWithSuffixes())
 		}
 	}