Parser: support 'alloc' vars

[ejrgilbert]

Fixes: #135

Parser:

• Extend parser to support new alloc keyword
• Test that alloc, alloc report, report alloc all work correctly

Rewriting:

• Logic to allocate the alloc variables as globals as emitting a probe at a match site
• Logic to handle report alloc variables (make a new lang_features/report_vars.rs to put this in one place)

Wizard:

• Logic to create the $alloc function for alloc variables that reserves memory
• Logic to load all alloc variables at the start of a probe's body
• Logic to save all alloc variables at the end of a probe's body
• Logic to handle report alloc variables

Test:

• (wizard) variables that are only alloc
• (wizard) variables that are only report
• (wizard) variables that are both report AND alloc
• (rewriting) variables that are only alloc
• (rewriting) variables that are only report
• (rewriting) variables that are both report AND alloc