Remove Failure CompileError

[myreen]

Currently, the compiler is allowed to return Failure CompileError
for any input program. At present, these compiler errors can only
occur when a code label is too far from the current program location,
i.e. the jump offset that needs to be encoded in the instruction just
does not fit the instruction encoding.

This issue is about removing such compiler errors completely by:

1. annotating each instruction that might cause such an encoding
   error with two registers that can safely be used as temporaries
   when computing the target address

2. adjust the lab_to_target compiler so that it makes use of
   these temporaries

3. adjust the semantics of LabLang and StackLang so that these
   instructions delete the registers mentioned in temporary
   register annotations

I suspect the word_to_stack compiler knows which two registers are
available to be used as temporaries on the StackLang side.

[myreen]

Note that this issue goes against #186.

[myreen]

A potentially less intrusive approach could be to leave the compiler
implementation as is, but instead construct a compiler configuration
transformer that given a normal compiler configuration produces a new
configuration where two registers are reserved (unusable by CakeML's
register allocator). These reserved registers can then be used to
implement "smart" encoders for instructions that require unusually
large code offsets.

One could prove that, given such a tweaked compiler configuration,
the compiler can never return Failure CompileError.

Use of such defensively written compiler configurations might not have a
performance impact on architectures that provide more than 16 registers,
since CakeML is unlikely to be able to make effective use of that many
registers anyway.

[sorear]

What's going to happen if I feed code that would be >4GB compiled to a 32-bit backend? Also, this seems to be in contradiction with #544.

[xrchz]

This should be taken into account by #1080 and will be superseded by that.