s/vlen/vl/

Closes #701.

Closes #702.

Add notes for vl=0 behavior of instructions that write scalars

Resolves #703

Each specification can be in one of the following states:
Development
Stable
Frozen
Ratified

In order to avoid confusion around which specification version should be
implemented we add the current state as well as a description of that
state to the preamble.

Signed-off-by: Stephano Cetola <scetola@linuxfoundation.org>

* vstart needs explicit initialization by software at reset

* Simpler fix that preserves usefulness of original commentary

First release candidate for v1.0 for public review.

defining prestart, inactive, active, body, and tail sections, and
corrected discussion of what elements might be updated.
Closes #689.

Add space between VType's operands

Closes #687.

vm=0 encodings should be reserved for mask-logical instructions

Changed assembler mnemonic of unordered floating-point reductions `vfredsum` and `vfwredsum` to `vfredusum` and `vfwredusum` respectively.  Older names kept as aliases.
Closes #681.

Clarified that mask load and store instructions operate as if EEW=8, EMUL=1 with reduced vector length.
Closes #665.

Closes #652.

Allow mask-agnostic tails to be written with the value that would have been produced with a maximum vector length.
Closes #639.

Closes #640.

Closes #612.

Made clear that implementations can raise illegal instruction exceptions if EEW not supported for address offset in indexed vector memory instructions.
Closes #599

Closes #587.

Removed the "write 0" request for vtype.  Can only be written via  vsetvl and that definition gives correct behavior.
Closes #575.