[CS] added sys.net API

some functions missing
not shure about blocking...
Merge remote-tracking branch 'origin/development' into cs/sys/net
document haxe.macro.Context
dammit
skip over unsafe casts when making constant expressions, but add them back afterwards
do not mute types when checking abstract casts
add std types to module context before calling onGenerate callback (closes #1904)
always filter empty abstract implementation classes (closes #1885)
[Php] handle CONTENT_ prefixed headers in Web.getClientHeaders (closes #1883)
fixed Context.definedValue returning "null" string
do not double-declare vars to keep java happy
push @:coreApi metadata on abstract to implementing class
handle @:coreApi on abstracts
add @:exhaustive only to enum matches
Merge branch 'development' of https://github.com/HaxeFoundation/haxe into cs/sys/net
Merge pull request #1898 from J-d-H/cs/sys/net

[CS] added sys.net API