Update internal ABI to zero/sign-extend where necessary only

[dcodeIO]

Related issue: #38

Previous behaviour was:

• When returning a small integer (i8, i16 etc.) from a function, make sure it is properly wrapped
• When assigning a small integer to a variable, make sure it is properly wrapped
• When performing an operation that might overflow, wrap after performing that operation

New behavior is:

• When returning a small integer (i8, i16 etc.) from an exported function, make sure it is properly wrapped
• When assigning a small integer to a global, make sure it is properly wrapped (see comment below).
• When performing an operation that requires properly wrapped values, wrap before performing that operation if necessary
• When an expression of a specific kind does not technically overflow, do not wrap its result
• Remember wrap states of locals throughout flows, and use this for tagging their respective get_locals .

[dcodeIO]

One more missing piece is remembering wrap status of locals thoughout flows. Without, occurences of get_local reset the wrap status to unwrapped, leading to quite a lot unnecessary re-wraps as can be seen in the fixtures.

This can't be done for globals because these are shared between functions, so I decided to wrap set_globals on assignments so code using globals can consider them properly wrapped. There are usually more gets than sets, so this should reduce more unnecessary wraps than it introduces.

[dcodeIO]

Ok, I think the most common cases should be handled now, but there are more opportunities along the road. Thinking of merging and adding an issue to track additional optimizations - maybe?

[MaxGraey]

Great idea. Also Alan working on souper branch which could be cover other optimization scenarios when it landed