Allow resetting to previous error stacks; Fixes to toplevel variable evaluation

[Affonso-Gui]

The function reset takes an optional argument to select which level it should reset (throw). However, in current implementation the stack always wind back to level 0, independently of the argument given.

Example below.

1.eus$ (let ((a 1)) (error "1st error"))
Call Stack (max depth: 20):
  0: at (error "1st error")
  1: at (let ((a 1)) (error "1st error"))
  2: at #<compiled-code #X66045f8>
eus 0 error:  1st error in (error "1st error")
2.E1-eus$ (let ((b 2)) (error "2nd error"))
Call Stack (max depth: 20):
  0: at (error "2nd error")
  1: at (let ((b 2)) (error "2nd error"))
  2: at euserror
  3: at euserror
  4: at (error "1st error")
  5: at (let ((a 1)) (error "1st error"))
  6: at #<compiled-code #X66045f8>
eus 0 error:  2nd error in (error "2nd error")
3.E2-eus$ (eval-dynamic 'a)
1
4.E2-eus$ (eval-dynamic 'b)
2
5.E2-eus$ (reset 2)
6.eus$ (eval-dynamic 'a)
*unbound*
7.eus$ (eval-dynamic 'b)
*unbound*

With this fixes, (reset 2) throws 2 and goes back to level 1:

1.eus$ (let ((a 1)) (error "1st error"))
Call Stack (max depth: 20):
  0: at (error "1st error")
  1: at (let ((a 1)) (error "1st error"))
  2: at #<compiled-code #X64dfdc0>
eus 0 error:  1st error in (error "1st error")
2.E1-eus$ (let ((b 2)) (error "2nd error"))
Call Stack (max depth: 20):
  0: at (error "2nd error")
  1: at (let ((b 2)) (error "2nd error"))
  2: at euserror
  3: at euserror
  4: at (error "1st error")
  5: at (let ((a 1)) (error "1st error"))
  6: at #<compiled-code #X64dfdc0>
eus 0 error:  2nd error in (error "2nd error")
3.E2-eus$ (eval-dynamic 'a)
1
4.E2-eus$ (eval-dynamic 'b)
2
5.E2-eus$ (reset 2)
6.E1-eus$ (eval-dynamic 'a)
1
7.E1-eus$ (eval-dynamic 'b)
*unbound*

@inabajsk

[Affonso-Gui]

@k-okada Please review

[Affonso-Gui]

Added commit to solve the following problems in toplevel variable evaluation:

• Cannot evaluate a symbol that is also fbound
• Evaluation of unbound symbols leads to unclear response: 32512

With this changes:

1.eus$ 10
10
2.eus$ *
10
3.eus$ * 2 2
4
4.eus$ ls
c               Makefile.Cygwin    Makefile.IRIX5      Makefile.LinuxSH4.2    Makefile.SunOS5.X11R6.1
comp            Makefile.Cygwin32  Makefile.IRIX6      Makefile.Linux.thread  opengl
geo             Makefile.Cygwin64  Makefile.Linux      Makefile.rgc           README
image           Makefile.Darwin    Makefile.Linux64    Makefile.SunOS4        README.rgc
l               Makefile.generic1  Makefile.LinuxARM   Makefile.SunOS4.sub    tool
Makefile        Makefile.generic2  Makefile.Linux.ppc  Makefile.SunOS5        xwindow
Makefile.Alpha  Makefile.IRIX      Makefile.LinuxSH4   Makefile.SunOS5.i386
0
5.eus$ guiga
sh: 1: guiga: not found
Call Stack (max depth: 20):
  0: at #<compiled-code #X52d7e80>
eus 0 error: unbound variable guiga in #<compiled-code #X52d7e80>
6.E1-eus$ 

Note that

1. * is symbol-evaluated when there are no trailing arguments and function-evaluated otherwise
2. unbound variable errors are appropriately raised on evaluation of unbound symbols
3. unix evaluation remains the same for valid commands

[Affonso-Gui]

@k-okada @furushchev @mmurooka Accepting comments.

[Affonso-Gui]

Changed base to cl-compatible for testing features

[Affonso-Gui]

Closing in favor of #366 and #367