Candidate for 6.0.0 release

[kushti]

This PR contains ErgoTree 6.0 features and fixes along with activation code.

It contains following features and fixes:

• SOption[T] (de)serialization support

issue: #659
PR: #990

• Fix for downcasting BigInt to Long fails

issue: #877

PR: #1007

• Accumulate ErgoTree deserialization cost

issue: #846

PR: #938

• Fix SubstContants: serialize ErgoTree size

issue: #994

PR: #995

• Support Header (de)serialization

issue: #969
PR: #972

• Header.checkPow method

issue: #958
PR: #968

• Box.getReg method

issue: #416
PR: #1015

• Serialize SFunc in TypeSerializer (higher-order lambdas support)

issue: #847
PR: #1020

• New Numeric methods

issue #1 : #486
issue #2 : #1006
PR: #1017

• Global.serialize method

issue: #988
PR: #989

• Global.fromBigEndianBytes method

issue: #993
PR: #1013

• New collection methods

issue: #1004
PR: #1010

• Improve collections equality

issue: #909
PR: #1011

• Lazy evaluation of default in Option.get and Coll.get

issue: #906
PR: #1008

• Context.getVarFromInput

issue: #978
PR: #1016

The roadmap for 6.0.0 and instructions for contributors can be found at ergoplatform/eips#100

Helper PRs:

• [6.0] Improved and correct testing of newFeature test cases #1001 (only testing scope is touched)
• [6.0] Common SMethod and MethodCallSerializer new features #1002