proc: only format registers value when it's necessary

[aarzilli]

proc: only format registers value when it's necessary

A significant amount of time is spent generating the string
representation for the proc.Registers object of each thread, since this
field is rarely used (only when the Registers API is called) it should
be generated on demand.

Also by changing the internal representation of proc.Register to be
closer to that of op.DwarfRegister it will help us implement #1838
(when Delve will need to be able to display the registers of an
internal frame, which we currently represent using op.DwarfRegister
objects).

Benchmark before:

BenchmarkConditionalBreakpoints-4   	       1	22292554301 ns/op

Benchmark after:

BenchmarkConditionalBreakpoints-4   	       1	17326345671 ns/op

Reduces conditional breakpoint latency from 2.2ms to 1.7ms.

Updates #1549, #1838

[aarzilli]

I realize that the amd64DwarfToName map isn't used anymore, except to build it's inverse map (amd64NameToDwarf) and so we could remove. However I'd like to keep it around since it will eventually be used to implement #1838 (we'll need a way to put a name on registers for which we only have a dwarf register number).

Also, I explored the possibility of a much more radical change where the backend specifies the DWARF register number directly (instead of the thing we have now where we find the register number by using the register name). Technically it's possible, however for the gdbserial backend we only have the register names so it would simply push the ugly (register name → register number) table down, so it isn't really worth it.

Finally the extra register names that get added manually in the constructor of amd64NameToDwarf are alternate names for some registers that mozilla rr uses.

[derekparker]

Nits and questions, otherwise looks good.

[derekparker]

LGTM