Merge branch 'feature/kostyantyn/traces' into 'master'

Stack traces for assertion failures

Related to issue #14 
Replaces !63

This merge request suggests a change that produces stack traces on assertion failures. Without this functionality it is difficult to investigate issues causing E-ACSL assertion failures: the previous design reported only line and file numbers where the assertions have failed and terminated executions using `exit(1)`. 

Changes made by this merge request:
- Added an interface for executing shell commands from within the C RTL (`e_acsl_shexec.h`). This interface is required for running `addr2line`, a `binutils` tool that translates addresses into line and file numbers.
- Added an interface to the GLIBC backtrace function so stack traces could be generated.
- Updated C RTL string header to add several support functions.
- Updated C RTL assert header to enable trace generation and also to raise abort signals rather than end the executions via `exit(1)`.

See merge request !66