Add preprocessor macros

[imrehorvath]

Add initial implementation of AVRASM2 style preprocessor macros. It is not yet a full implementation. However it does support the following.

.device atmega328p

#define EIGHT (1 << 3)
#define SQR(X) ((X)*(X))

ldi r16, EIGHT
ldi r16, SQR(4)

#define FOOBAR subi
#define IMMED(X) X##i
#define SUBI(X,Y) X ## Y

IMMED(ld) r16, 1
SUBI(FOO, BAR) r16, 1

IMMED(ld) r16, SQR(SQR(1+1))

Regression test provided, all passing.

cd tests/regression && ./runtests.sh

Testing p36-invalid-inequality-operator
PASSED

Testing pr16
PASSED

Testing pr31
HEX file ok
EEPROM file ok
PASSED

Testing pr36-invalid-equality-operator
PASSED

Testing pr36-valid-binary-operators
HEX file ok
EEPROM file ok
PASSED

Testing preprocessor-macros
HEX file ok
No expected EEPROM file found
PASSED

Testing test1
HEX file ok
EEPROM file ok
PASSED

Testing test2
HEX file ok
EEPROM file ok
PASSED

8 tests, 0 failed