use allocas for variables

[edg-l]

Depends on #67

The previous version used always registers, which will make some future things not quite possible, using allocas is the way to avoid some limitations of SSA and makes easier to implement branches.

Ultimately though, the simple.con program gets compiled to the same code.

The add_plus_two function:

0000000000000030 <add_plus_two>:
  30:   8d 44 37 02             lea    eax,[rdi+rsi*1+0x2]
  34:   c7 44 24 fc 02 00 00    mov    DWORD PTR [rsp-0x4],0x2
  3b:   00 
  3c:   c3                      ret