libexecstream

CMake-ready libexecstream 0.3 library.

Changes from original libexecstream 0.3

• CMake scripts to build the library.
• Sources moved to src/ and include/libexecstream/ directories.
• Removed unnamed namespaces (Some implementation types from that namespaces are being exposed to the API). Eliminates -Wsubobject-linkage GCC warning.
• Use const char* instead of char* for string literals. Eliminates -Wwrite-strings GCC warning.