Optimize performance of extension functions defined on Source when called on a Buffer

[fzhinkin]

Some extension functions defined on a Source (like readString or readCodePointValue) prefetch data into the source's buffer and then delegate further processing to buffer-specific functions. When invoked on a Buffer, these functions still do all these unnecessary prefetching actions. Instead, such calls should be delegated to a Buffer directly.

Potentially affected functions are:

• Source.readCodePointValue()
• Source.readString()
• Source.readHexadecimalUnsignedLong()
• Source.readDecimalLong()
• Source.indexOf