add new SOFTFORK condition

[arvidn]

Enabling this is a hard fork, and it allows future conditions to be added as soft-forks and have non-zero cost.

The new condition is HARDFORK. The opcode is 90. It requires at least one argument, which is the cost of the condition (specified in 10 thousands). All other parameters are ignored.

In memepool-mode (NO_UNKNOWN_CONDS), any SOFTFORK condition is an error, as expected.
In consensus mode (the default), any SOFTFORK condition is silently ignored, but the cost is applied.

[coveralls-official]

Pull Request Test Coverage Report for Build 5084415251

• 14 of 16 (87.5%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 60.824%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/gen/validation_error.rs	0	2	0.0%

Totals
Change from base Build 5083277513:	0.1%
Covered Lines:	812
Relevant Lines:	1335

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💛 - Coveralls

[richardkiss]

Looks good.