Experimental support for symbolic opcodes in ASLp.

Opcode semantics generation now accepts an "opcode" template, for
example:

```
ASLi> :sem A64 0x122:9|sh:1|imm:12|0xa4:10
Decoding instruction A64 0x122:9|sh:1|imm:12|0xa4:10
bits ( 64 ) imm__3 ;
if eq_bits.0 {{ 1 }} ( sh [ 0 +: 1 ],'0' ) then {
imm__3 = ZeroExtend.0 {{ 12,64 }} ( imm [ 0 +: 12 ],64 ) ;
}  else {
if eq_bits.0 {{ 1 }} ( sh [ 0 +: 1 ],'1' ) then {
imm__3 = ZeroExtend.0 {{ 24,64 }} ( append_bits.0 {{ 12,12 }} ( imm [ 0 +: 12 ],'000000000000' ),64 ) ;
}  else {
assert FALSE ;
}
}
__array _R [ 4 ] = add_bits.0 {{ 64 }} ( __array _R [ 5 ],imm__3 ) ;
```

* symbolic opcodes (#4)

Experimental support for symbolic opcodes in ASLp.

Opcode semantics generation now accepts an "opcode" template, for
example:

```
ASLi> :sem A64 0x122:9|sh:1|imm:12|0xa4:10
Decoding instruction A64 0x122:9|sh:1|imm:12|0xa4:10
bits ( 64 ) imm__3 ;
if eq_bits.0 {{ 1 }} ( sh [ 0 +: 1 ],'0' ) then {
imm__3 = ZeroExtend.0 {{ 12,64 }} ( imm [ 0 +: 12 ],64 ) ;
}  else {
if eq_bits.0 {{ 1 }} ( sh [ 0 +: 1 ],'1' ) then {
imm__3 = ZeroExtend.0 {{ 24,64 }} ( append_bits.0 {{ 12,12 }} ( imm [ 0 +: 12 ],'000000000000' ),64 ) ;
}  else {
assert FALSE ;
}
}
__array _R [ 4 ] = add_bits.0 {{ 64 }} ( __array _R [ 5 ],imm__3 ) ;
```

* fix symbolic field handling by `CommonSubExprElim` (#5)

  Fix a problem with the treatment of symbolic field variables by
  common-subexpression-elimination.

  When a common subexpression has been identified, it needs to place it in
  the statement list. It does this by placing it in the first position
  after all its dependencies have been assigned. The problem comes because
  a common subexpression that depends on a symbolic field won't ever find
  an assignment to the symbolic field.

  This is what the algorithm was probably supposed to do, but as they
  explain in a comment what they actually do is place the definition of
  the common subexpression in the first place after all its dependencies
  have _appeared_ (whether assigned to or not). Therefore in practice,
  common expressions involving symbolic fields would end up placed after
  their first use.

  This PR fixes it by:

  * Passing a list of globals into CSE which are treated as predefined
  * Appending symbolic fields to the global identifier set

* resolve merge conflict markers

* dis_decode_entry_sym

* sym_and_bits: use sym_slice and sym_append_bits

* tests/test_asl.ml: fix call to dis_decode_entry