Add IPv6 support to IPAddress

[sgryphon]

Some suggested code to add support for IPv6 to the IPAddress class. (Issue #168 )

The public interface changes are minimal:

• add a type() getter and corresponding enum
• add three new constructors: one for 16 octets, empty with type specifier, and byte pointer with type specifier
• add constant for IP6ADDR_ANY (similar function as IPADDR_NONE)

The changes are backward compatible. All of the IPv4 test cases continue to pass without any changes. i.e. Any IPv4 code will continue to get the same outputs for a given inputs.

Test cases added to cover IPv6.

Note: Whilst backwards compatible, this change does increase the memory footprint from 4 bytes to 20 bytes (assuming the enum compiles as an underlying int32_t, I think).

[CLAassistant]

All committers have signed the CLA.

[codecov-commenter]

Codecov Report

Merging #169 (bcc5233) into master (e03b653) will increase coverage by 0.10%. The diff coverage is 97.27%.

@@            Coverage Diff             @@
##           master     #169      +/-   ##
==========================================
+ Coverage   96.00%   96.11%   +0.10%     
==========================================
  Files          13       13              
  Lines         827      953     +126     
==========================================
+ Hits          794      916     +122     
- Misses         33       37       +4     

Impacted Files	Coverage Δ
api/Common.h	100.00% <ø> (ø)
api/IPAddress.cpp	97.15% <97.22%> (-0.81%)	:arrow_down:
api/IPAddress.h	100.00% <100.00%> (ø)

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