Split ext_ptw into pieces and add documentation

It was somewhat annoying that CHERI would have had to track both
successful and erroneous occurrences within the same type.  Break the
failures out to their own form and, while here, attempt to write down my
understanding of the extension's information flow through vmem.

Just because they're not present in the SV32 PTE doesn't mean that
models necessarily need to interpret them as zeros; any constant will do
just fine.  This allows extensions (like CHERI) that have both RV32 and
RV64 versions to define more standard idiomatic interpretation to the
bits within the PTE extension field.

A kinder, gentler splitting of ext_access_type's ReadWrite

This redoes https://github.com/rems-project/sail-riscv/pull/57 without
nearly as much excitement.  We sill want it for CHERI, so that we can
signal from the instruction to the PTW whether we are prepared to load a
capability (or will strip any tags that we load) and whether the store
will (or might) set a tag.

Thanks to Prashanth Mundkur for several improvements.

Otherwise there's no use for PTW_Ext_Error. This is required for
sail-cheri-riscv to be able to make use of its own page table
exceptions. The alternative to this is to pass the ext_ptw to
translationException, but given PTW_Ext_Error exists it seems this was
the intended way, and aligns with how access faults vs page faults are
distinguished.

Fixes #58.

- make opam package include files required for building rmem

Fix mtval when store gets bad PMP

* New functions to simplify float NaN detection

* Remove unnecessary intermediate values

Now that we have simpler function f_is_NaN

* FMIN/FMAX should return canonical NaN if both operands are NaN

Fixes #52.

* Simplify logic for FMIN/FMAX

Spec says "If only one operand is a NaN, the result is the non-NaN
operand." So no need to distinguish SNaN from QNaN here.

Fixes #51.

Fixes #50.

* Add {} so I can add a new variable here next

* Create new variable which I will soon reuse

* Plumb in the access type to effectivePrivilege

So I can use it to fix #47 next.

* Instruction fetches should not be affected by mstatus.MPRV

Fixes #47.

* Remove now-redundant privilege calculation

Switch floating-point comparisons to using softfloat to avoid missed corner-cases in hand-rolled helpers.