Add global variables to Cretonne IL.

See #144 for discussion.

- Add a new GlobalVar entity type both in Python and Rust.
- Define a UnaryGlobalVar instruction format containing a GlobalVar
  reference.
- Add a globalvar.rs module defining the GlobalVarData with support for
  'vmctx' and 'deref' global variable kinds.

Langref:
    Add a section about global variables and the global_addr
    instruction.

Parser:
    Add support for the UnaryGlobalVar instruction format as well as
    global variable declarations in the preamble.

lib/cretonne/meta/base/formats.py

@@ -10,7 +10,8 @@ from cdsl.formats import InstructionFormat
 from cdsl.operands import VALUE, VARIABLE_ARGS
 from .immediates import imm64, uimm8, ieee32, ieee64, offset32, uoffset32
 from .immediates import boolean, intcc, floatcc, memflags, regunit
-from .entities import ebb, sig_ref, func_ref, jump_table, stack_slot
+from . import entities
+from .entities import ebb, sig_ref, func_ref, stack_slot
 
 Nullary = InstructionFormat()
 
@@ -19,6 +20,7 @@ UnaryImm = InstructionFormat(imm64)
 UnaryIeee32 = InstructionFormat(ieee32)
 UnaryIeee64 = InstructionFormat(ieee64)
 UnaryBool = InstructionFormat(boolean)
+UnaryGlobalVar = InstructionFormat(entities.global_var)
 
 Binary = InstructionFormat(VALUE, VALUE)
 BinaryImm = InstructionFormat(VALUE, imm64)
@@ -42,7 +44,7 @@ FloatCompare = InstructionFormat(floatcc, VALUE, VALUE)
 Jump = InstructionFormat(ebb, VARIABLE_ARGS)
 Branch = InstructionFormat(VALUE, ebb, VARIABLE_ARGS)
 BranchIcmp = InstructionFormat(intcc, VALUE, VALUE, ebb, VARIABLE_ARGS)
-BranchTable = InstructionFormat(VALUE, jump_table)
+BranchTable = InstructionFormat(VALUE, entities.jump_table)
 
 Call = InstructionFormat(func_ref, VARIABLE_ARGS)
 IndirectCall = InstructionFormat(sig_ref, VALUE, VARIABLE_ARGS)