FIX(inst-table.adoc) asciidoctor-pdf Prawn::Errors::CannotFit.

Constrain vms*f to require vstart = 0

Rationale is similar to that for vmpopc/vmfirst, and avoids need for
a constraint on `vd`.

Closes #201.

Replaced widening multiply-subtract instructions with widening signed/unsigned multiply-add instructions.

Made integer single-width multiply-subtract opcode names and function match the scalar/vector floating-point operation names/functions.

Canonicalized ordering of multiply-add operands in various instruction descriptions.  No change in functionality.

Add note about vsetvl with rd=x0; explain why it is useful

Resolves #199

Closes #194.

Closes #193.

Closes #192.

Closes #169,

Closes #198

Changed behavior of mask-logical instructions to write boolean values (zero-extended 0 or 1 value) instead of performing bitwise logical operations.
Also changed name of pseudo-instruction vmmv.m to vmcpy.m to signify that it no longer does a bitwise copy.
Closes #190.

Fixed vmerge synatx to treat mask input  as regular vector, and then to treat unmasked variants as  real instructions instead of pseudo-instructions.
Closes #189.

Closes #165.

Closes #163.

Partially addresses #162

Permit implementations to trap vstart values they can't produce

Fix vadc.vim syntax typo.

Now need to use widening reduction  opcodes to get widened result.
Integer widening reduction instructions specify signedness.
Closes #158.
Closes #161.