[Ival] Restructuring ival, split into 4 files

Closes #613

See merge request frama-c/frama-c!2192

Shares the implementation of the checks between the two files.

Specialize string functions on usage

See merge request frama-c/frama-c!2381

In int_val, removes specific code handling empty sets.
Modifies the function [diff_if_one] that created a set with too many elements
before removing one element.

Fixes #613.

Int_val uses the limit provided by int_set.get_small_cardinal to create
sets or intervals.

In Ival, bottom is now a special constructor.

They respectively implements the semantics of small integer sets and integer
intervals.

Exports 4 new functions in ival:
- is_int and is_float
- is_small_set and project_small_set

[e-acsl] remove copy-visitors from E-ACSL pre-stages

See merge request frama-c/frama-c!2492

[e-acsl] remove unnecessary code in Prepare_ast corresponding to type 'position'

[e-acsl] ensure that terms of the E-ACSL ast are never shared before code injection. This invariant is required for E-ACSL typing soundness