Lots of mecha changes and additions.

Added constructible Phazon parts and circuits to R&D machines.
Added a Phazon construction path.
Nerfed the Phazon greatly.
 - equip slots down to 3 from 4
 - step_in up to 2 from 1
 - toxin punch damage greatly nerfed (from ~80 tox damage to ~20 per hit)
 - research levels required increased (materials 7, bluespace 6, power 6, emp 6, data 5)
 - research levels from parts reduced
 - no longer gets an RCD and gravcatapult upon creation

Fluffed out the Phazon in general (sounds, descriptions).

Grammar fixes to mecha construction process - added a bunch of missing periods and articles.
Changed Durand construction to require a super capacitor and phasic scanner instead of advanced ones.

Buffed the exosuit extinguisher (now has 1000 water instead of 200).
Buffed the exosuit plasma generator (now consumes fuel at half the speed and doesn't burn through masses of plasma in minutes).
Moved the cable layer tool from medical_tools.dm to tools.dm (why even).
EDIT: Fixed #4972, now mech sleepers can be resisted out of.
EDIT: Fixed a bug where Gygax/Durand new welding sprites during construction looped infinitely.
EDIT: Fixed no click delay on aliens hitting mechs.

Buffed Ripley's default armour (brute from 20% to 40%, bomb protection 80% instead of none).
All Ripley variants now have the same lights_power 7 (from Default Ripley's 6, and Death Ripley's 60 which basically created a fully lit square around it).
EDIT: Moved Ripley Firefighter chassis from Exosuit Equipment list to Ripley parts list.
The default Ripley can now be augmented with goliath armour plates, up to 70% brute protection (plates drop if Ripley is killed).
Goliath-plated Ripleys get a different sprite and examine-description.