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Add support for substring on the left of expressions

not complete, the gpr for SDL must call opengeode to generate Ada with
all input files in one go, not one call per file

Due to the AST storing now the full path of the ASN.1 files

as per ISS request

And remove -lgnat from Makefiles, this is not needed anymore with recent
GNAT versions

when flattening model, the access to fields was not properly prefixed
for variables declared in substates

e.g. if a branch only contains informal or ANY decisions,
generate "null;" to avoid an empty code block in Ada