More models for the c string library

[MatthewARinehart]

Why

As a Manticore user, I want to work with binaries that frequently perform string manipulation. Since symbolic string manipulation can be computationally intensive, I'd rather use a high-level model of the string API that eliminates the overhead from symbolically executing every instruction.

Acceptance Criteria

Manticore provides built-in models for the following string functions:

• [x] strcpy - copy a string (#1681)
• [x] strlen - get string length (#1752)
• [x] strncpy - copy part of a string (#1761)
• [ ] strcmp - compare two strings (#1762)
• [ ] strncmp - compare parts of two strings (#1763)
• [ ] strcat - concatenate two strings (#1764)
• [ ] strncat - concatenate one string with part of another (#1765)
• [ ] strchr - string scanning operation (#1766)
• [ ] strrchr - string scanning operation (#1767)