extending codegen to support instantiations of arbitrary templates

[ab9rf]

at the moment i'm looking at something akin to this:

<template-instance template-name='name'>
contents are then either
<type-parameter>
a simple type
or
a compound (which will be redued to a T_ type and used as the parameter)
or
a subordinate template-instance (for nested template instances)
</type-parameter>
or
<type-parameter type-name='...'/> which is a shorthand
this is repeated as many times as appropriate
</template-instance>

<stl-variant> would then be an alias for <template-instance template-name='std::variant'>

there's a further extension (to be done later) for dealing with function types (which is what the type parameter of std::function is) that i'm thinking will be repurposing the existing vmethod tag

we'll probably eventually want to be able to declare templates as well; there is at least one template type in g_src now -- we're ignoring that it's a template because all of the instances are binary-compatible so we can pretend they're all the same type, but this can't be counted on being the case forever, but this isn't immediately needed so that's a future thought

anyway, thoughts welcome none of this is set in stone, and i got nowhere when i tried a first pass at implementing this yesterday so i'm still at square one in this regard

Originally posted by @ab9rf in #686 (comment)

[lethosor]

a compound (which will be redued to a T_ type and used as the parameter)

My guess is that this can be a lower priority. We can always define our own types, and defining types inline in template arguments isn't valid like it is for class fields - neither of these compile:

std::vector<struct { field a; }>
std::variant<struct { field a; }, struct { field b; }>