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:

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