* mem_t: Throw an error if zero-sized memory is requested

If for some reason the user requests a memory size of 0 megabytes, print
a useful error message.

* Check for overflow in memory size

If the user passes in a large enough memory size (-m) that the size in
bytes doesn't fit into size_t, catch this error in the make_mems function.

Add support for program buffer of size 2

Also add an implicit ebreak after the program buffer. This is not part
of the spec, but hopefully it will be.

Simply not writing the register was not a conformant implementation.

Closes #159.

https://github.com/riscv/riscv-isa-manual/commit/a62e76cb16eb508199f74632eb8bf263739f25a3

A regression caused any instruction with rd=x0 to not be emitted.

Support for non-contiguous hartids

It's supplanted by the hypervisor extension, which doesn't use the privilege
encoding of 2; it still looks like supervisor (i.e. 1).

Resolves #152

Put HTIF in the device tree

I wanted to actually put the address of the HTIF into the DTS, but that
seems to be a bit too much work: since the HTIF addresses are just
defined in an ELF file it's a bit awkward to make that work.

Instead, I'm just putting a dummy HTIF key in the DTS.

* Fix commit-log for Q extension, and for RV32

The number of nibbles printed out now depends upon XLEN or FLEN,
as appropriate.

* Factor out FLEN calculation

Fix a bug that prevented bus_t from storing to, loading from,
or finding a device that existed at address 0x0.

Resolves: #135

If rd=rs1 or rd=rs2, the NaN check examined the wrong value.

commit 85c40db2 introduced a NAN dependency but did not include the math.h header

Implement Q extension

Fix debug reset.

ndmreset now resets all harts (instead of just the current hart), and
hartreset resets the selected hart (instead of being ignored).

Specifically, don't print out the execution count if the same
instruction is executed by different harts.