Added to_int, is_int and string_from_int

[EliasC]

This commit extends the String library with support for translating
to and from ints. Tests in src/tests/encore/stdlib/string.enc have
been extended.

[albertnetymk]

• the if statement has two identical branches in string_from_int
• is_int doesn't seem like a library function
• strtol could be used to parse a string with error detection in to_int
• it's good to include a test case for integers mixed with strings, sth like 12abc

[EliasC]

Reacted to all of @albertnetymk's comments. Code is much better now. I also made sure that the string tests are run with make test

[albertnetymk]

Looks great. The stdlib tests are not integrated in the same manner as the rest, but that could be addressed in anther PR. Merging now.