Compatibility with Nim v1.6.x, Nim v2.0.x, Nim v2.2.x

[mratsim]

A significant amount of upstream bugs and regressions have been fixed and backported related to statics, generics and templates which Constantine uses heavily.

• [Regression] static integers in quote do nim-lang/Nim#22947
• [Semi-regression] C code contains backtick`gensym nim-lang/Nim#23711
• Regression: compiler crash on static inference nim-lang/Nim#23755
• Regression: ICE genCheckedRecordField with unsafeAddr and macros nim-lang/Nim#23784

This PR works around the last one that prevent Constantine from compiling on devel:

• Semi-regression (?): generic typeclass not matching nim-lang/Nim#23547

In summary the following cause a type mismatch:

func mulCheckSparse*(a: var QuadraticExt, b: static QuadraticExt) {.inline.} =

but the following works

template mulCheckSparse*(a: var QuadraticExt, b: QuadraticExt) =

However there were 2 template symbol generations issue:

• one that was fixed in [backport] fixes #23711; C code contains backtick`gensym  nim-lang/Nim#23716
• one that exists in Nim v2.0.x but not in devel/v2.2.x

In C, the symbol generated contains invalid backticks like t`gensym1234

Fortunately, another way to workaround this is to tag the variable as {.inject.} which makes the library supported in 1.6.x, 2.0.x and 2.2.x