Extensively refine code for these three serial I/O examples

[pat-rogers]

README.md

• Add new readme indicating that this host program is only intended for the
  stream-based demo, and explaining how to use it

host.adb

• Add much more comments in the header block explaining how to use this program and
  why it is needed
• Parameterize the host port number instead of hard-coding it

demo_serial_port_blocking.adb

• Use normalized procedure names for Send and Receive, for consistency with
  other versions
• Add comments re: baud rate and other settings required

demo_serial_port_nonblocking.adb

• Better name for local procedure
• Add many more comments re: settings

demo_serial_port_streaming.adb

• Replace source code in comment header block with location of the actual
  code and project file.
• Add comment re: serial port settings

peripherals_blocking.ads

• Peripheral should be a constant
• COM now references the USART designated by Peripheral

peripherals_nonblocking.ads

• Same as for peripherals_blocking.ads

peripherals_streaming.ads

• Same as for peripherals_blocking.ads

serial_io-blocking.ads

• Type Serial_Port now designates a USART rather than type Peripheral
• No longer provide call-through routines for the Serial_IO package's
  Initialize_Hardware and Configure since client can easily call them. Better
  separation of concerns as a result.
• Change signatures of internal routines to be access-to-USART

serial_io-blocking.adb

• Internal simplifications reflecting change to access discriminant now designating type USART
• Remove code for Initialize_Hardware and Configure, per removal from package spec

serial_io-nonblocking.ads

• Type Serial_Port now designates a USART rather than type Peripheral
• No longer provide call-through routines for Serial_IO packages
  Initialize_Hardware and Configure since client can easily do so, and simplifies this package
• Change signatures of internal routines to be access-to-USART

serial_io-nonblocking.adb

• Internal simplifications reflecting change to access discriminant now designating type USART
• Remove code for Initialize_Hardware and Configure, per removal from package spec
• Use short-circuit control forms in ISR since don't need to check both conditions, should be
  a little faster

serial_io-streaming.ads

• Type Serial_Port now designates a USART rather than type Peripheral
• No longer provide call-through routines for Serial_IO packages
  Initialize_Hardware and Configure since client can easily do so, and simplifies this package
• Change signatures of internal routines to be access-to-USART

serial_io-streaming.adb

• Internal simplifications reflecting change to access discriminant now designating type USART
• Remove code for Initialize_Hardware and Configure, per removal from package spec

serial_io.ads

• Procedure Initialize_Hardware's formal parameter is no longer an access parameter
• Procedure Configure's first formal parameter is now access to USART

serial_io.adb

• Change to procedures' signatures as per spec
• Procedure Configure now uses distinguished receiver syntax for USART routines, much cleaner