Evaluate memory region expressions

[xarantolus]

Hello,

I've recently been in the situation where I wanted to parse a linker script like the following:

MEMORY
{
  FLASH : ORIGIN = 0x08000000, LENGTH = 8K

  RAM : ORIGIN = 0x20000000 + 256K, LENGTH = 640K - 256K
}

The crate doesn't support evaluating expressions like in the origin and length field of RAM; the parse function returns an Ok result with an empty vector, which was unexpected for me.

I added code that directly calculates fields like LENGTH = 640K - 256K for memory regions. A lot of operators are not yet supported, because I wasn't quite sure how to handle their edge cases, but this provides a base that works for most expressions one would use in these memory declarations.

All tests continue to pass. Please feel free to point out any issues :)