Improve NTX4 and NTX5

[benber86]

There were several issues with NTX4 and NTX5 (some referenced in #35):

• Calls to external contracts were not taken into consideration, but a pure function may call a pure function on another contract (likewise for a view function)
• Likewise NTX4 or NTX5 could be raise even if the function was doing an extcall and was therefore neither pure nor view
• The way memory reads was defined was too broad, function arguments, constants, immutables and locally declared variables were all defined as memory reads even though they don't actually read from storage and can be used in pure/view functions.