Increase base AC on heavy armour items

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After reading ​http://takkaria.org/junkyard/angband-plan.txt I felt agreement with all of it except the "too many weapons not enough armour" bit. I agree that too many weapons get dropped in relation to armour, and tweaking the drops in favour of armour is fine ... but I think there are still several heavy armour items which are all but useless without the elvenkind ego. I propose fairly large increments to base AC to make it worth carrying the extra weight, doubling the scale from 4-40 to 4-80. i.e. Soft Leather stays at 4, but Adamantite Plate doubles to 80. Even that might not be enough protection to make heavy armour worth wearing, but I hope some testing will tell us otherwise.

Ideally the AC-per-unit-weight should increase as the weight increases, so I will play around with slightly reducing the weights as well, to see if we can achieve this.

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Also worth revisiting the mage-caster mana penalty at the same time - to prevent quite such a large proportion of armour items being unusable by mage casters. My initial thought is to slow down the rate at which SP are lost, e.g. perhaps 1 per 2lb or similar. Will work up some proposals.

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Also worth doing some improvements to DSMs at the same time, like reduced recharge times.

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[b0b2d91] (SVN r2008)