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 instruciton 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.

Third-party test suite now also includes:

• golang.org/x/crypto/curve25519
• filippo.io/edwards25519
• github.com/oasisprotocol/curve25519-voi
• github.com/ericlagergren/lwcrypto

Changelog

• tests/thirdparty: add ericlagergren/lwcrypto by @mmcloughlin in #219
• tests/thirdparty: add oasisprotocol/curve25519-voi by @mmcloughlin in #220
• tests/thirdparty: golang.org/x/crypto/curve25519 by @mmcloughlin in #222
• tests/thirdparty: package metadata by @mmcloughlin in #223
• tests/thirdparty: use shallow clone by @mmcloughlin in #224
• tests/thirdparty: add filippo.io/edwards25519 by @mmcloughlin in #227
• tests/thirdparty: add skip option by @mmcloughlin in #228
• all: AVX-512 by @mmcloughlin and @vsivsi in #217

Full Changelog: v0.3.1...v0.4.0