This adds new `Int8`, `Int16`, `Int32` and `Int64` data types
to the compiler, thus working towards properly specified integer
types as discussed in #1048.
In addition, the following changes / corrections are made:
* Support casts from `Char`, `String`, and `Double` to all integer
types (and back). This fixes#1270.
* Make sure that all casts to limited-precision integers are properly
bounds checked (this was not the case so far for casts from `String`
and `Double` to `Int`)
* Add a thorough set of tests to make sure all bounds checks work
correctly for all supported casts and arithmetic operations
