debugger: symlist usability + symbol table extensibility

[dave-br]

This addresses issue #6655 (symlist command usability), plus adds a bit of plumbing for future extensibility.

Symlist command usability

The distinction between global and CPU symbols is unclear to most who don't read the source, so this makes symlist more natural:

• symlist with no arguments displays all global and :maincpu symbols, with clear header text for each list. At the bottom, prints helper text to make user aware of the cpu form
• symlist <cpu> works as it has before, only displays symbols for specified CPU

Symbol table extensibility

This pr includes some changes in anticipation of features that add new kinds of symbol tables. Example: the source-level debugging pr (#13444) adds symbol tables for local and global variables that appear in the debugged source code. The following were originally part of the source-level debugging PR, so merging this will shrink that PR somewhat.

• Add an enum field to symbol table for its 'type', for prettier printing from symlist.
• symlist now traverses symbol table chain completely. No noticeable difference from before, but this will make extensions to the chain work automatically
• Eliminate hard-coded limit (1000) on number of symbols to print per symbol table

Sample session

A CoCo with Speech-Sound cartridge (thus, two valid CPUs).

>symlist

**** CPU ':maincpu' symbols ****
a = 0
b = B
cc = 90
curflags = 90
curpc = 10C
cycles = 24  (read-only)
d = B
dp = 0
lastinstructioncycles = 18  (read-only)
logunmap = 1
pc = 10C
s = 7F1E
totalcycles = 24E077  (read-only)
u = ABEE
x = 402
y = A00E

**** Global symbols ****
beamx = 85  (read-only)
beamy = D9  (read-only)
cpunum = 0  (read-only)
frame = A1  (read-only)
temp0 = 0
temp1 = 0
temp2 = 0
temp3 = 0
temp4 = 0
temp5 = 0
temp6 = 0
temp7 = 0
temp8 = 0
temp9 = 0
wpaddr = 0  (read-only)
wpdata = 0  (read-only)
wpsize = 0  (read-only)

To view the symbols for a particular CPU, try symlist <cpu>,
where <cpu> is the ID number or tag for a CPU.


>symlist :maincpu

**** CPU ':maincpu' symbols ****
a = 0
b = B
cc = 90
curflags = 90
curpc = 10C
cycles = 24  (read-only)
d = B
dp = 0
lastinstructioncycles = 18  (read-only)
logunmap = 1
pc = 10C
s = 7F1E
totalcycles = 24E077  (read-only)
u = ABEE
x = 402
y = A00E


>symlist 0

**** CPU ':maincpu' symbols ****
a = 0
b = B
cc = 90
curflags = 90
curpc = 10C
cycles = 24  (read-only)
d = B
dp = 0
lastinstructioncycles = 18  (read-only)
logunmap = 1
pc = 10C
s = 7F1E
totalcycles = 24E077  (read-only)
u = ABEE
x = 402
y = A00E


>symlist 1

**** CPU ':ext:multi:slot3:ssc:pic7040' symbols ****
a = FF
b = 0
curflags = D0
curpc = F2D1
cycles = 0  (read-only)
genpc = F2D1
lastinstructioncycles = 9  (read-only)
logunmap = 1
pc = F2D1
sp = 4C
st = D0
totalcycles = 24E062  (read-only)


>symlist 2
Invalid CPU index 2

[dave-br]

Just a gentle bump... merging this fixes an issue (i.e., improves symlist usability), and helps to shed some more code off of my larger source-debugging PR. Happy to answer any questions.