[breaking] Upgrade to Go 1.23, fix ReadStringAt to take maxLength as …

…arg (#23)

This is needed because StringBuilder.Grow(n) function now may grow
the underlined buffer longer than the value of [n].

Add support for Flush only when mapped region is modified

* Remove exported interface

* Rename struct from FileImpl to File

* Change interface and struct names

* Add functions to read/write string data

* Breaking API changes

* ReadUint64At, WriteUint64At don't return error
* Remove using `golang.org/unix` package

* Breaking API changes, satisfy io.ReaderAt & io.WriterAt interfaces

* Use golang.org/unix package
* Remove dependency on unsafe and syscall package
* Fix tests to use MS_SYNC for flush
* Full code coverage using unit tests

* Introduce IMmap interface to ease testing

* Add support for Go modules
* Update documentation

This is the first release for the mmap library.

It currently provides basic functions to map, unmap, read, write and advice.

Currently, supported OS -
  * Darwin OS
  * Linux

Currently supported Architectures -
  * Little Endian architecture