Improve ToString/ToStringE performance

[ganigeorgiev]

Currently ToString/ToStringE always attempts to perform pointer dereference before the type checks which adds significant overhead when casting non-pointer values (for pointers it is also not very optimal because the reflection of errorType and fmtStringerType would be evaluated on every ToString/ToStringE call).

This PR:

• moves the pointer check in the default clause and calls ToStringE with its underlying value
• updates the existing tests with equivalent checks for pointers

Below are some basic benchmark results from my local environment (the best from 3 consequent runs):

1. Plain value

   func BenchmarkToString(b *testing.B) {
       for range b.N {
           cast.ToString("test")
       }
   }

   // Old:
   BenchmarkToString-4     19877439          60.69 ns/op
   
   // New:
   BenchmarkToString-4     282848110         4.364 ns/op
   

2. Pointer

   func BenchmarkToString(b *testing.B) {
       v := "test"
       for range b.N {
           cast.ToString(&v)
       }
   }

   // Old:
   BenchmarkToString-4      4797831         287.2 ns/op
   
   // New:
   BenchmarkToString-4     10021204         216.1 ns/op
   

[CLAassistant]

All committers have signed the CLA.

[sagikazarmark]

LGTM, thanks