Optimizations to the calltree under ExpandPropertiesLeaveEscaped - PR #…

…6128

Context
Property expansion has been identified as one of the hot spots in project evaluation and ExpandPropertiesLeaveEscaped alone
accounts for almost 20% of overall evaluation time of simple projects.
Several optimizations have been made to the calltree under ExpandPropertiesLeaveEscaped. Notably:
- Unnecessary and counter-productive string pinning has been removed.
- A slow strchr-like function has been replaced with a simple call to String.IndexOf.
- Series of ifs have been replaced with a switch.
- Allocation of List<object> results has been eliminated.
- Allocation of temporary substrings extracted from the expression has been eliminated.
- The combined performance win is 10% in ExpandPropertiesLeaveEscaped, so close to 2% for evaluation overall.