Improve performance for string concat

[lucasccordeiro]

ESBMC-Python can verify this Python program in a few milliseconds while ESBMC-Python-CPP takes many seconds:

str1 = "Hel"
str2 = "lo"
str3 = str1 + str2
assert str3 == "Hello"

str4 = "Wor" + "ld"
assert str4 == "World"

assert str1 + "lo" == "Hello"